I use mine for emulation, homebrew, and ebooks.
Homebrew:
noiz2sa is a fantastic shooter
Vektar, the homebrew one, is great as well.
Both of these are a must have on your SD card IMO.
I am slowly working my way through what is available.
Emulation:
Neo Geo with the gngeo2x is great for full speed with sound all of my favorites in my hand Neo Geo games! I have almost 50 games running and I'm enjoying every minute of it! I am clocking it at 266 mhz.
Genesis with the DrMDx is great!!
Hu6280 is my Turbo Grafx/PC Engine emu of choice. My favorite system by the way.
I run SMW on snesgp2x.
ebooks:
I have the collected works of “Doc Savage” on cd that I got off of ebay. Very good scifi-adventure stories from the 1940s. There are about 200 books, I think. I used to have some of the paperbacks years ago that I picked up in a used book store. I just change them to text and put them on my SD card. I have one on my card that I am reading now.
Since I got my GP2X around the 4th of July, I have done all of my gaming on this thing! I have not been able to pick anything else up since.