There is Flashcarddb.com which is free and so far looks like the best option. There is the very simple and good looking cramberry.net too (not free though).

To anyone out there reviewing Japanese (or anything else) online, what do you use? Please.

P.S: RevTK won't work since I want to be able to review vocabulary and sentences as well.

Ankiweb? I see no reason to use anything that's not Anki.

Anki!

Thanks for the suggestions. I tried Anki Online and it's exactly what I was looking for. Featureless, simple, streamlined, perfect.