There is a whole list of info on Beta Testing including services offered for doing this on some of the links.
I cannot recommend anyone as it is not something I currently do with the products produced where I am, our systems are bespoke, so there is no advantage for us to beta.
Also Alan Richardson, www.compendiumdev.co.uk , did an excellent talk at eurostar 2003, "Be a Better Tester, Be a Beta Tester", on beta testing. I know a number of excellent testers who participate in Beta programmes, but not through any specific companies.
Agile Testers of the World UNIT!
Does anyone know of (or better yet have experience with) a firm that does beta testing - by that I mean a firm that can recruit and qualify testers, incent testers if appropriate, manage the beta process, communicate test tasks, run post test survey, compile and report the test results. The project I have in mind would be testing a plug-in to Outlook in enterprise and small/medium business environments.