From the moment that I got my Treo 600, I wanted to play Nintendo GameBoy Color games on it. The OMAP CPU should be more than strong enough for the average gameboy game, and the 3 MEGs of RAM should also suffice. Getting the games running was a bit tricky - here is how I did it:

Step one - get the emulator
In order to play Gameboy games on a Treo, you need an emulator. I chose LJP because it keeps most of its data on VFS - get the PSI installer here, and install it onto your Treo. Afterwards, run the program and wait while it installs all neccessary data to your SD card.

Step two - get the ROM’s
This is the difficult part - you need to download the contents of gameboy modules(called ROM’s) from somewhere(sometimes called a ROM world). Nintendo is very territorial about them, so don’t ask me where to get them from.

Step three - get the ROM’s onto the Treo
Stuff the Treo’s SD card into a card reader and copy the .gb or .gbc files into the following folder:
/palm/programs/ljp/gb/roms

Step four - launch LJP
After having stuffed the card back into the Treo, launch LJP. Switch to Gameboy mode(dropdown at the left, choose GB), set up the controls(Config button), choose the ROM and click Launch. Your Treo now becomes a Gameboy - enjoy!

In case you don’t believe me, look at the following Videos(at Google Video):
Tetris DX on Treo 600
1942 on Treo 600