If you have not come across it before, web2logo.com provides an extensive listing of companies in the social computing and Web 2 space. There seems to be approximately 1000 companies listed there in one form or another. Some logos are repeated (e.g. Google in different versions) but that's rare. Clicking on a logo will give a description from Web2list, site traffic data from Alexa, and current Technorati-tracked blog activity for each of them. It's hard to say if this data is accurate but it does give an idea of which ones are doing decent enough to watch.
Community and social computing
with Tags: company X