[Jabdoa2]

Sounds like a very useful service. I just signed up. Did you consider to inform new users that the service is financed by donations? You seem to focus on "always free" which is nice but I guess most people are also ok to pay some money if your service is useful to them. I guess this might drive some more donations if people knew that you need money. Also a banner like Wikipedia (maybe a bit less annoying) would work.

[ihid]

Hello. You'll get an email after a day telling you about it, and you'll get a banner after you've submitted a couple of exercises :)