Nihongo Help is an excellent aid for testing your knowledge of Japanese to further ingrain it into your mind.

Version 1.1 includes Hiragana and Katakana. Generate a random sequence of characters to see if you can read or pronounce them all correctly, or generate a random sequence of pronucniations to test whether you can write the corresponding characters from memory.

Note: Nihongo Help is not meant to teach you Japanese, it's just meant to help re-enforce concepts for people who are self-studying. If you're looking for something to teach you Japanese, I recommend Human Japanese. Not free, but not expensive either... and very well written. You can think of this application as a supplement to it.