I have Kotoba for reference and Japangoggles (if and when it works) to find out kanji I don't recognize. But for a learning tool, I use
かな・かな. It's a vocabulary builder, with a bunch of hirakana floating on the screen. You must form words with them by moving them around, any combo will do. There are different levels, where you must make words corresponding to certain theme, etc. It's the first time I've encountered the concepts "learning in Japan" and "fun" in connection with each other.
