I've been using projecteuler.net to learn a bit of programming, and I think this is a great format to learn other stuff (learning by doing). I'm wondering if there's any other sites similar to this, that in essence is a list of challenges to learn a discipline.
This is a tricky one, as problem based learning is suited to programming challenges as it is very easy to submit your solution online - other subjects such as gardening would be quite hard to submit online! :)
However, you could check out this site http://www.mrsoshouse.com/pbl/pblin.html
It has various resources on Problem Based Learning along with various Activities, Problems and Projects on a range of subject.
In my opinion: using http://www.wikihow.com can teach you various things in "learning by doing" methodology.