There was a mild debate on Stack Overflow today regarding the posting of solutions to the puzzles on other sites like Project Euler. The debate was started by this question. Several people disagreed with the questioner, but I'm on the fence.
I love sites like Project Euler, Top Coder, and the Python Challenge for the puzzles they provide. At least 90% of the enjoyment I get from them is in solving the problems myself, with no outside help. I have to admit though, that at least a small part of the enjoyment is in competing with other people who are trying to solve the same puzzles.
I don't mind at all when people give hints (especially the non-programming hints that seem to be required to advance to the next level of the Python Challenge). I do get a little bit annoyed, though, when I see people posting entire solutions to the puzzles from other sites. Like it or not, competition is a component of the enjoyment that people get from programming puzzle sites like these. Having others get full solutions to the puzzles removes that part of the challenge.
I know that there's little that the administrators over at Stack Overflow can do about people posting solutions to puzzles on other sites. People can easily rephrase a question so that it isn't obviously a Project Euler puzzle (for example). I also know that the solutions to many of these problems can be found with a quick Google search.
That leaves it up to the Stack Overflow community. As I said, I see nothing wrong with posting questions asking for hints and tips to questions from puzzle sites. I'd like to see more of them. What I do see wrong, is when people post full solutions to these problems. Having a full solution takes away the learning experience that one might have otherwise enjoyed. I think that taking away a learning experience goes against the spirit of both Project Euler and Stack Overflow.