Printable hiragana and katakana flashcards like the ones we use in class.
Website with movies that show how to write each katakana.
Wikipedia page with information about katakana's history and use
Katakana practice site that lets you choose which columns you'd like to practice. (iPhone version available)
The Ultimate Katakana Challenge - how quickly can you put the characters in order if they are moving all over the screen?