Generally, we take a pretty dim view of design contests and things that cause people to do work on spec, but there are some websites where you can battle in public for graphic arts money, which seems a bit more interesting.
Some of you might want to use sites like these to build your portfolios, get some design exercise, and see how you shape up next to the competition. If you're in a place where US$300 is a lot of money, you might even make rent.
Read the fine print. Often this sort of work will result in "work for hire" art, which means the copyright is sold with the design. When you sell your copyright, you can't sell that design again, but you can usually display it in your portfolio or hang the original on your wall.
If you have a choice, licensing the "right to use" your work is a better plan. That way when the license expires, the work belongs to you again. Of course, legal and copyright law likely varies by country.
