this looks really helpful for those who want to learn
there are also some learning games out there
http://lrnj.com/ - Slime Forest. It's like one of those old school rpgs, but you attack by memorizing the kana
http://www.kicl.info/ - Knuckles in China Land. A much more fleshed out game, and my personal favorite. Helps in teaching you kana, kanji, and vocab while giving you a storyline to boot.
Please everyone post more links if you've got them!