Pretty much everything up to n64 I believe. I think there are ps1 emulators as well, but last I saw one it didn't run perfectly....that was years ago though. If you just wanted emulators you wouldn't even have to upgrade the hdd most likely as they typically have a small file size.
286/386 -
DOSXBox
A 286/386 emulator that emulates old dos games such as commander keen, jazz jackrabbit, all old apogee games and basically any other game that can be run in dos.
Adam/Colecovision -
AdamX
Better collecovision emulator than Koleko and it also emulators Adam.
AGI Sierra -
SarienX
A Sierra AGI interpreter engine that will play many of the old Sierra adventure games including Leisure Suit Larry 1, Kings Quest 1, 2 and 3 and others.
Amiga -
WinUAEX
An amiga emulator by xport, better than uaex
Amstrad CPC -
ArnoldX
An Amstrad CPC emulator by xport.
Apple IIgs/e/c/+ -
KegsX
Plays Apple IIgs/e/c/+ at good speeds
Arcade -
MAMEoX128 Plus! or
BenderMameOX HSFII Edition
An updated version of MameoxVMx. This emulator is also one of the main reasons for getting a 128MB ram upgrade (this emulator works fine on 64MB XBoxs too). MAMEox 128 Plus! has many updates including updated NeoGeo/CPS drivers, IPS support, and much more. It even has complete RomCenter and ClrMamePro DATS to audit your ROMS. BenderMameOX HSFII Edition features added drivers for Hyper Street Fighter II The Anniversary Edition and, best of all, light gun support has been fixed!
Atari800/5200/130/320/XL/XE -
AtariXLBox
An Xport emulator that emulators atari 800/5200/130/320/XL/XE and runs all games perfect.
Atari 2600 -
Z26X
Better compatibility than Stella and as of v5 it now supports save states and rewind.
Atari 7800 -
Atari7800X
An
Atari 7800 emulator ported from
MESS to the XBox by Xport.
Atari Lynx -
Mednafenx-lynx
Replaces LynxBox. Better speed and compatibility overall. Also features the the rewind function.
Atari ST -
WinstonX
Currently the only Atari ST emulator on Xbox.
Chip-8 -
Chip8x
A port of
Chuit, a Chip-8/Super Chip-8 emulator, by Xport.
Commodore PET -
VicePETX
A Commodore PET emulator, and the only one around too.
Commodore Vic-20 -
Vice20X
The only and best Commodore Vic-20 emulator
Commodore 64 -
Vice64X
It has taken over Frodo-x and now becomes the best C64 emulator for xbox
CPS3 Arcade -
CPX3
Plays Capcom Play System 3 arcade games. Currently has some issues with slow down due to the limitations of the XBox hardware.
Daphne Laserdisc -
DaphneX
The best and only Arcade Laser Disc emulation.
Dreamcast VMU -
SoftVMU-X
A Sega Dreamcast
VMU emulator ported to the XBox by Xport.
*
note: this is a VMU emulator, NOT an actual Dreamcast emulator!!!
Intellivision -
BlissX
The best and only fully functional intellivision emulator.
Killer Instinct 1/2 -
KI-XXX (based on U64-X)
Emulates the games Killer Instinct 1 and 2. KI-XXX does not run from disc, but there is a version of this emulator by Likklebaer floating around that does. It is not available at the usual place though... and don't ask where to get it on the forum!
Mac Plus -
Xmac
Xmac is an XBox port of
minivac, a Mac Plus emulator. Ported to the XBox by Darcelf.
MSX/MSX2/MSX2+ -
BlueMSXBox
Better than FMSXbox giving you a better experience when playing MSX/MSX2 and MSX2+
Neogeo CD -
NeoCD-SDLx Unleashed
Emulates NeoGeo CD. Originally ported by Lantus, the "unleashed" version features ISO support thanks to GogoAckman
Neogeo/CPS1/CPS2 -
Fbaxxx or
FBAXXX Pro
Absolutely the best Neo Geo emu on Xbox. With the capability of playing the majority of Neo games almost flawlessly, AND being able to play Capcom's CPS 1 and CPS 2 arcade software, this is an essential emulator for your Xbox. FBAXXX is the primary emulator on which Gogo and the Logic Sunrise team work. FBAXXX Pro is based directly on FBAXXX and incorporates the latest drivers, support for more games/clones, and attempts to keep up with the latest ROM sets. As a result, FBAXXX Pro should only be used by individuals who don't mind updating some of their ROMS with virtually every new release. The original FBAXXX doesn't change ROM sets so each new version normally works with all the ROMS supported by previous version. Unfortunately FBAXXX is not currently available at the usual place. You will have to track it down yourself.
Nintendo 64 -
Surreal 64 XXX Beta 5
An update of the best n64 emulator for xbox surreal64. Surreal 64 uses 3 emulators into one so you can choose which emulator you want to play (1964, project 64, ultraHLE. The xxx version has better game speed and has been updated with the latest Rice plugins. Some people feel that Surreal64 FDB is more stable than the Surreal XXX releases, which is why it is also listed. But we all still wait for the official Surreal64 V2.0
Nintendo Game Boy Advance -
XBoyAdvance
Not only does this play GBA pretty damn well, but it will also play game boy and game boy color as well. On the GB and GBC you can also play split screen system link
Nintendo NES -
Mednafenx-nes
This emulator replaces FCEUX. Better compatibility overall and more accurate color palette along with additional features like "rewind".
Odyssey 2 -
OdysseyX
Port by xport that emulates the Odyssey 2.
Pokemon Mini -
PokemonMiniX
A port of Minimon, a Pokemon Mini emulator, by Xport.
ScummVMx - A emulator for several classic point-and-click adventure games such as Monkey Island, Sam and Max, Full Throttle and Beneath a Steel Sky.
Sega Genesis/32X/CD -
NeoGenesis or Xenesis
Choose either Neogenesis by xport or Xenesis by Carcharius, both support 32x and sega cd emulation though its said that xenesis plays sega cd better. Both basically have the same features but neogenesis has more configurable menus and has the ability to overclock sega genesis games. You should get both and choose which one you like.
Sega Master System / Game Gear / SG-1000 / SC-3000 / SF-7000 / MK3 -
MekaX
Emulates all the above Sega systems. Also supports lightgun for the master system.
Sinclair ZX81 -
Xzx81
A port of
xz81, a Sinclair ZX81 (aka TS1000) emulator. Ported to the Xbox by Darcelf.
SNK NeoGeo Pocket -
NeoPopX
NeoGeo pocket emulator that plays all game perfectly.
Sony Playstation -
PCSXBox
The only Playstation emulator for the XBox (excluding any run on top of Linux). Plays many games very well.
Spectrum -
DidntXSpectrum or
FuseX
Two Unix Spectrum emulators based off the same FUSE emulator source code. The DidntXSpectrum port of FUSE was done by Xport and FuseX by skratchy.
note: I don't have any personal experience with either or these two emulators, so if anyone has any input on anything I should add to their descriptions please let me (Thraxen) know.
Super Nintendo Entertainment System -
Zsnexbox
ZsnexBox is a port of the windows emulator ZSNES. It features fullspeed emulation, excellent sound, excellent compatibility, and many features the PC port doesn't even have. It supports the Xbox lightgun or the gamepad for Superscope games, force feedback/rumble on a per game basis, cheat searching, PAR/Game Genie support, 3 built in cheat databases with over 10,000 cheats/tips/codes, full HD support, DSP4 emulation, an enormous amount of options that can be tweaked, a built in music player supporting several classic video game formats (Genesis, SNES, NES, etc…), and comes with 3 excellent skins by Neverwill. This is the best SNES emulation experience to be found on any console. For the very few games that Zsnexbox doesn't support, xSNES9x is your next bext bet.
Texas Instruments 99 -
Classic99x
An XBox port of
Classic99, a Texas Instruments 99 emulator, by Xport.
Thomson TO7-70 -
Xthom
Xthom is an XBox port of Thom, a Thomson TO7-70 emulator by Sylvain Huet and Eric Botcazou. Ported to the XBox by Darcelf.
Turbo Grafx 16/PC Engine/Super Grafx/CD/SCD -
Mednafenx-pce
Replaces HuGoX. Vastly improved compatibity compared to HuGoX, especially CD games... most of which now run near perfect. Also features the rewind function and support for simulating 6-button game pads.
Watara Supervision -
SupervisionX
An XBox port of Potator, a Supervision emulator, by Xport. Description from Wikipedia: The Watara Supervision was a monochrome handheld game console, originating from Hong Kong, and introduced in 1992 as a cut-price competitor for the Nintendo Game Boy. It came packaged with a game called Crystball. The console had a slightly larger screen and larger buttons, and its games sold for far less than the Game Boy's. The games were not of high quality, and the console did not sell well. The original design for the console changed significantly through several iterations, and the last Supervisions were sold in 1996. The Supervision was marketed by several different companies around the world and under several different names, including Quickshot Supervision and Hartung SV-100.
Wonderswan -
WonderSwanX
Plays most games at decent speeds but i've still seen alot of games have a slow down in them.
Vectrex -
Xvectrex
A port of
vecx, a GCE Vectrex emulator. Ported to the Xbox by Darcelf.
Virtual Boy -
VirtualBoyX
A port of the Red Dragon Virtual Boy emulator. Ported to the Xbox by Xport.
X68000 -
X68000X
An xport port of the X68000
Edit: This list is dated, so there are most likely more out currently. This is a few years old.
. Emulators, a must I believe because they can all be accessed just like a pc, but on a BIG tv 
