Most lists of "up and coming" open-source companies are somewhat flimsy, essentially bought by PR firms. But this one from iTWire actually took some thought, and I think it gets things mostly right. The companies/projects profiled are Google, OpenMoko, rPath/XenSource, and Red Hat/Canonical (Ubuntu).
I thought the article's commentary on Google's Summer of Code was particularly enlightening, being a sometime Google critic myself:
What's most astonishing about the Summer of Code is that Google really do not clearly appear to benefit. Instead, they are using their own money to fund development of clearly defined and external open source projects. Personally, I'd have been ecstatic when a University student to have a paid vacation opportunity like this (but we didn't have the World Wide Web when I was a boy :-) Consequently, Google really do deserve acclaim for their tangible support in enhancing and promoting free and open-source software throughout the world, not to mention steeping new generations of graduate software developers in open-source ways.
Indeed. In fact, this is something that mostly holds true across the range of companies profiled: they're not completely self-centric. They do at least some code simply for the community.
If I were making my own list, I'd add a few more, like Digium, LoopFuse, and MuleSource, among others. But since I advise some of these, I won't.