I occasionally joke about the fact that I bought a PS2 and a DS a while back by saying that I'm using them to study Japanese. (Hit pause, grab dictionary, look up options, unpause...repeat as needed) However, it has actually come true, quite literally.
The Nintendo DS has such crossover appeal that a lot of people who you wouldn't normally expect to be "gamers" have picked one up. Japanese game companies are catering to this crowd by producing "games" that are meant to help you learn something. In addition to the ubiquitous "Brain Training" games (the original has at least two sequels here that may or may not reach the states), there are DS interactive cookbooks, DS English drill programs, DS "How to survive being a tourist in America/Korea/Germany/Thailand/etc.," and even a DS Japanese-English-Kanji dictionary. That would have been convenient had I not already had a Canon Wordtank.
But, I've picked up a couple of games to help me prepare for the Japanese test this December.

This is the first one I picked up, on recommendation from Doug. Unfortunately, it's too advanced for me -- I can't read the on-screen text all that fast. Still, hopefully I'll "grow into it." Or, if not, there's always Book-off...

This is the one I picked up today. It's aimed more at children, and it was published by the association that administers the official Japanese Kanji tests. My boss thinks I should take this test too, but I'd be uncomfortable sitting in the elementary school classroom with 3rd/4th graders, taking the same test as them.
I got this game for free, pretty much. More on that later.