For Nihongojira it's my desire to plug only the finer sites that can help in the quest to learn.
Another one I have found, thanks to those wonderfully contributors on the JPod forum is this little place: the JLPT Study Page.
It is specifically geared to the most widely (only?) recognised exam for students of Japanese. Lots of useful references and a system that links to the best grammar resource on the web, Tae Kim's, mean it is a great place to check your level. I'm pleased to see it also references Jim Breen's JDic and Meguro Language Centre.
That's pretty much all the places I recommend too.