I would say Usertesting.com is the best. This requires some thought into how to design the test cases, and what to ask for.
Bob Walsh's StartUpToDo has a function for requesting feedback, which returns results in a nice normalized form of points given in specific areas. This is a less open-ended form of feedback than Usertesting, but could be a good supplement.
The regulars at Joel Spolsky and Michael Pryor's BoS forum have often contributed mini-reviews. This may work better for you if you target a specific problem on your site, and if you contribute over at BoS from time to time.
Last but not least, do classic hallway usability testing with people you know.
There's a lot of advice here directing you to go to one of a number of generic places to get general feedback. Watch out for that! What you really need is to test your site with your customers. Use HackerNews if your customer base includes primarily software developers and startup founders. Use Reddit if your customer base are primarily redditors. And so on. What you really need to do is figure out where your customers reside, and go to them for the testing. That's the only way you're going to get high quality feedback which will help guide you on the path toward building your startup.
I recommend taking this opportunity to show it to some members of your target audience. If you can't target that audience precisely enough via UserTesting.com, just find a couple in person and sit them down in front of your computer. This is not a substitute for the opinions of web-savvy professionals, but if I had to choose one group to review my site, it'd be my target audience. After all, you're a web-savvy professional yourself, right?
I think the best is to first hang out and ask at the news hacker news irc room (#startups) and then later on go to ask on Hacker News proper... While you will get a lot of feedback about the UI there too, the great thing about Hacker News is that they will tend to also discuss your business model and marketing plans...
But as Brian said, you do need a thick skin...