Elance.com - This is a great place to post work, this company is used by over 100,000 businesses. This is a great place to find programmers, designers, writers, marketers, customer service agents and virtual assistants.
oDesk - This is a major resource for businesses looking to outsource to freelancers. They have over 400,000 service providers from over 100 different countries. They also have currently over 90,000 different businesses that use their bidding and posting network.