My son started taking Spanish this year, and I wrote FlashCards to help him study. The app lets you choose groups of words and study them in random order. It will tell you the word in one language and you have to type in the equivalent word in the other language. If you're right, it moves on to the next word. If you're wrong, it'll tell you without revealing the word. You can mark words one of two colors (maybe words you missed and words you had trouble with) and then study just the words you marked.
The trial version is limited to the built in set of Spanish words, but the full version allows you to use words from StudyStack.com. StudyStack is a nice website that has thousands of groups of words to quiz yourself with. It allows you to set up a free account and mark lists as ones you like (as well as upload your own). If you have a StudyStack account, you just need to enter your username into the FlashCards app, and it will download your favorite stacks for you.
The full version also allows you to create your own words in any set of left to right languages (sorry, right to left languages aren't supported by WP7 yet). So, if you want to do say German to French, and you've got some web space somewhere to place a text file, you can create your own list and pratice. Personally, I dug up my old Japanese word lists from back when I was studying Japanese and put them on a file I could study. I probably won't go all out in studying Japanese again, but it's nice to have the flash cards around for when I've got a few spare moments to kill.
See the How To link below for instructions on the format of the wordlist. If you want to start from the builtin spanish list, it's here. My Japanese list comes from many years studying Japanese in the University of Washington's "Japanese Program for Professionals" program. Although I've changed around the order of the words, these were all the words I typed in during that time. You can get that list here. If you have bought the full version of the app and want to use the Japanese list, you can start the app, tap "Actions" and then "Get Words From The Web." Then type in the address of the word list there (ie "http://www.mystikeep.com/apps/flashcards/japanese.txt"). Sorry that's a long thing to type on the device, but at least you only have to do it once.
Instructions on how to create your own word lists
Screen shots of FlashCards in action
Copyright Michael P. Calligaro