OLPC Deb XO
For now see this page and either flash the nand flash image or install the bootable USB/SD image or download the image haxwithaxe has made here and follow the USB/SD instructions from the DebXO page. If you use haxwithaxe's image you Must expand the partition to at least 2GB.
1) Boot up the XO with the boot media inserted or if you flashed the nand then just boot up.
2) If you used haxwithaxe's image then skip this step else read on. The default image is missing some rather important bits like the terminal activity and the means to install it. To install any activity go here and write down the url for the .xo file of the activity you want and then get to the virtual console by pressing ctrl+alt+<first gradiated circle key> (the gradiated circle keys and the volume/brightness keys are mapped to the function keys). Then wget the .xo from the url without the http:// (damned if I know why :/ ), run "sudo apt-get install unzip", then unzip the .xo and place the resulting <activity name>.activity folder in "/home/olpc/Activities/". To get back to Sugar press ctrl+alt+<a gradiated circle key> ... i say "a" because it's one of them but i can't tell which one. Just press each one successively until Sugar pops up.
3) Go wild with it it's full on Debian :P If you want you can follow the Soundmodem and make obnoxious noises :) or one of the other OLPC specific how tos on this wiki.