The main ones are Beaver and caps-a-holic who of course have the same goal as us (helping consumers compare releases) but in a different way, hence why we link to them as we think it is beneficial to our users to check out visual comparisons as well as 'paper specs' comparisons. Both have their positives and negatives of course. The amount of work in getting screengrabs etc means Beaver and caps-a-holic have to spend a lot more time per comparison and are limited to the discs they have in their possession. We can of course add a lot more releases in a quicker time scale, but rely on people submitting accurate specs and we are essentially 'paper based' going by guidelines we have to select a 'winner', but unable to confirm for people which release might have the better transfer etc.
I always tell people to use a mix of ourselves, Beaver, and caps-a-holic, and look at reviews from Basen to make sure they are buying what they are truly looking for. I think all four sites should be immensely proud, not only of what they have achieved, but how we are all going stronger than ever! For example, our visitor numbers are up between 15-25% each month on previous years despite us being 18 years old!
As James said though, ofdb is a great resource for German discs! Also movie-censorship is a great source for censorship (and melonfarmers).