In Germany there are a couple of platforms offering mostly mid to long-term contracts: freelance.de or freelancermap.com for example. freelancermap.com seems to be a little more international.

As a freelancer / contractor myself I wish there were more website for that, but the quality of all these project marketplaces aren't great at all.

Besides, mid/long-term projects pose some risks for the freelancer and often the company, too.