I went through the submissions to Stack Overflow's 99designs logo contest and noticed that some of the non-winning logos were very good. Can I use some of them on my website? If so, are they free to use, or would I have to pay to use them?
Short answer: No.
Long answer: Read the 99designs faq on "Who owns what, and when?". The people who designed the logos still own the copyright (except for the winner which is now owned by SO), so the only way you could use them would be to track down the individual creators & negotiate directly with them.
Ya! Go sell them! Make money off of the poor people that made them!
Seriously though, would you like someone using your work without permission?
And what could you possibly need them for?