I'm wondering if there exist any community driven site, to join two kind of people...
Initially, I thinked in common "download" sites, but many of them don't allow beta software to be uploaded. Also, testing or startups sites are outside of what I'm looking for.
So, if Youtube exists for people who like to see new things (videos) and for people who want to shown what they've created and receive opinion, there should be same for software.
Somehow I get the feeling this is not what you are looking for, and were probably well aware of these two communities prior to my post. These two communities are still going to be your best bet in terms of finding beta testers. You might similarly ask this question on stackoverflow.com/, programmers.stackexchange.com/, asp.net, or some other similar application development website. I somehow get the feeling that the website you are looking for has already been developed.
Try http://startuplift.com its public but its all about getting users to test your side and give valid and constructive feedback. The interesting part of this website is that you pay $19 dollars and then you can award this to the best feedback. I got some great insightful and deep feedback from this site