One of my favourite website is www.zadane.pl
(czyta się -mnej więcej- dablju, dablju, dablju, dot, zadane, dot, pe, el)
I really like it because it helps me with homework. On this website, you can write exercise (but you have to have points), then somebody write answer. I think that's so helpful, if you can't do some exercises. People collects points by writing answers, essays, logging everyday and inviting other people. You can meet new buddys. There is points ranking of users.