http://topcoder.com/

TopCoder is a crowdsourcing website dedicated to offering top quality work from designers, developers, and data scientists at an affordable price. With over one million members, it’s easy for buyers to find what they’re looking for. Registration is free for all users.

Freelancers can join challenges and submit ideas and designs. Challenges include prizes up to several thousand dollars or as low as zero. Freelancers can filter challenges to find opportunities in their areas of expertise, or else view them all and start participating.

Specialization: TopCoder specializes in crowdsourcing web development.