I'm developing a web based personal project that will hopefully be used by thousands. Obviously prior to releasing it in the wild as Beta I was wondering, are there any communities out there that will test my web app and provide feedback? Free would be great, but I'm open to suggestions.
People here perhaps? And users at other stackexchange sites.
Maybe also people from a non-technical background, if, as you say potentially 1000s of users, some of those are likely to be non-technical and will have different issues regarding usability.
You might want to think of some incentives to motivate people to test your site: rewards, founder membership etc.
And also a criteria to test against.
Maybe you're already aware of the established user-acceptance testing discipline so forgive me if I've told you what you might already know so I won't say too much, except to say there are books, discussions and evangelists on the subject. O'Reilly - oreilly.com the book publish would be a good place to look for more ideas.
Check out this book: "Effective UI: The Art of Building Great User Experience in Software" http://oreilly.com/catalog/9780596154783/
Speaking from experience. Find your target audience. Find people you know, people who will be honest. You have to have a pain in the butt friend who would love to tear your app to shreds. Though this answer seems light, watching a user use your app is the most beneficial thing i have ever done. My favorite test is to sit them down and give them no instruction. Figure out what they WANT to do.
I use three stages of testing:
Alpha testing is primarily done among the developers. It sounds like you're in this stage. Here people are responsive from a technical angle, they can identify errors and know what causes them along with fix recommendations. Programmers.SE might be the crowd you're looking for here.
Beta testing is done among your target audience. Advertise a beta period, keep it to a manageable size and have a public wiki / bug / issue tracker available.
This is an optional step for developing applications. Here you would advertise to your target audience and beyond a potential release. Freeze the version number and consolidate features here. Limit work to bug fixing and minor UI issues.