Dunno if this little mini-game has been linked here before, but it's a handy trainer for counting, knowing when you're in tenpai, when to ron, when you can go riichi, etc. It's all in Japanese, but ロン is ron, ツモ is tsumo, リーチ is riichi, and カン is kan. That's all you really need to know to play. (It also lies to you frequently, and will chombo the heck out of you if you ron when you shouldn't!)
http://www.gamedesign.jp/flash/bamboo/bamboo.html
I saw it linked on the reachmahjong forums earlier today, and spent a good hour or two driving myself nuts. I figured I'd pass it on.