DSReader
DSLibris
DSOrganize
Well, for me, I don't use the PDA stuff of DSO yet, but it's integrated web browser is pretty damned good for a small device, as is it's IRC client, but I've never been much of an IRC fan...
Also, because of the integrated browser and hence HTML rendering, DSO also makes a good local HTML file viewer via the file browser. Also does a good job with plain text files.
The other two are pure e-text readers, but I'm not sure which I prefer yet. Both lack in the bookmarks department, as in too few of them. DSO doesn't seem to support bookmarking either via www browser or via viewing from file browser...
[EDIT]
I'll toss in some gamey stuff too, as it's also homebrew
lameboy/Goomba GB/GBC emulation
SNESemulDS SNES emulation (best of the lot, but not great lots of glitches on lots of games, and needs manual tweakings)
Tower of Dagur(sp?) nice little jRPG, battle system sucks, and AFAIK magic system is still unimplemented
Powder roguelike, similar to hack, no class selection
NethackDS nethack for your DS
SNESemulDS WILL use your 3in1, etc. as expanded memory, but I haven't noticed that it makes a difference for small games as I believe that it's just used for mapping of larger carts...
[/EDIT]