I'm working on a website that I'll need beta tested soon. It's a startup. Where should I go to get a good pool of testers? It would also be great to have a good pool of hackers who can show security holes before it's released.
You can get great feedback at http://news.ycombinator.com make sure to post in the "Ask" section.
Also, you can try to submit your startup to http://betali.st You should also submit your startup to as many blogs and bloggers as possible.
I have been fairly successful finding beta testers in LinkedIn and Twitter, by providing a link to a landing page that has some information about my web app. In LinkedIn I just ask the question about people wanting to do beta testing and there is usually a good number of people willing to try.
I usually use unbounce.com to create landing pages.