The Nintendo DS, though most people think of it as a portable game system, is actually a great bargain for students of Japanese. Even if you're not into video games, buying a DS and the dictionary software I suggest can save you a lot of money. But there is also a lot of kanji testing software for the DS, the best of which I will outline here.
The Nintendo DS is region-free, so you can play Japanese games on your US or European DS with no problems. Since the DS is cheaper in the US than in Japan, even if you're going to Japan you should buy the US DS.
Kanji Training Games
There are three excellent kanji practice games for the Nintendo DS.
This is the newest kanji game to hit Japan. Unlike all of the others, this one focuses on developing proper writing skills, rather than writing the characters "just well enough" for them to be recognized. Read my review of Kanji Calligraphy Training.
Second, Otona no Kanji Renshu (Adult Kanji Practice, features an excellent writing practice feature. Players first write the reading and then the kanji, using a common phrase or saying as their guide. Although the phrases can be difficult, since all of the kanji except the one you are supposed to write have furigana, it is easy to look up new words in the dictionary.
The second kanji game is more difficult and is intended as a study tool for kanji tests. However, it features many different types of problems, including reading problems (where one reads passages rather than just a few words or phrases), stroke counting, radical identification, and other problems.漢検DS2 (Kanken DS2)
Dictionaries | Denshi Jisho
Nintendo has published excellent dictionary software for the Nintendo DS (based off of the Genius dictionaries). Find out more on the dictionaries page.