There's a lot to explore on the homebrew side of the Wii, but it depends on what you want to do with it.
There's a laaarge list of Emulators that open the doors to many systems, such as FceUltra RX, SNES9x RX, Genplus GX, mGBA, and then you have Retro Arch, which is a collection of emulators working under the same "hood".
You have ports of games such as Doom (and other games under that engine), Quake 1 & 2, Duke Nukem 3D, Hexen II, Outrun and more, and also a number of homebrew games you can check.
Some applications I use quite often are these:
GCCM, a Game Cube Memory Card Manager if you use real ones.
Save Manager Gx to do the same for Wii games.
CleanRip, to back up your own GC and Wii discs.
Again, there's a lot to see. You would do well to check Priiloader too, as it can be quite useful and help you out of a number of issues.