Beta Testing as a Service

Hi all,

Does anyone know of a service that, for a fee and with well defined parameters, will gather a bunch of users to perform beta testing on a product?

So, say there's a browser extension that someone has developed, and she wants to get, say, 15 people to install it, use it, play with it, and provide useful feedback on it (more than just,"this is awesome" or "this sucks") - feedback that can be used to shape the product.

I've heard of Betalize, but, visiting their site, it's very hard to tell how it works and how to launch what they call a "contest". Does anyone know if they're still in business? Anyone know of a worthy competitor?

This seems like an obvious idea and a service just about every startup would need. But I haven't seen anything like this. But maybe one of you has, and so I'd appreciate any insights you can offer.

Thanks very much!