One such website in the news these days is utest.com. They work on crowd sourcing model and I have seen rave reviews about it. (I am assuming here you mean functional testing only and not "white box" testing)
Check its review here (Disclaimer: I am not affiliated to uTest.com in any way)
You may also what to look at Mechanical Turk. (I am not affiliated with Amazon but sure do like them!)
I agree that uTest is a great resource. Also, you should definitely ask people to check out and test your site on:
http://news.ycombinator.com/ http://linkedin.com/answers I've used both sites to do just that and the response has been great. Below is a link to the feedback I got in a few occasions:
You could also try 99tests.com - recently launched and a pretty good tool to find out bugs.