Brain training on your internet browser
Did you know that the Nintendo DS isn't the only system that can train your brain? In Japan, Sega created their own mildly-successful edutainment games that required you to do math at the speed of light. AZ92 from the PlayStation forums is hosting a few Flash demos of the game, formatted for the PSP. You can use m.pspfanboy.com to play these short demos, directly from your PSP.
[Update 1: First server ran out of bandwidth. Updated links. Thanks, Crazy_Chris]!
