WFMU out of New Jersey is pretty much the big Freeform radio station, and embraced webcasting relatively early. They've had a few tough times financially, though, and will occasionally have sales and fundraisers – one of the biggest is their yearly record fair, where record geeks from all around come to look and trade rare stuff. Laura Cantrell is one of many who has a show – hers being based around old and rare country stuff. They've also got novelty music shows, kids music shows, rock shows and most other types of show you could think of. It's pretty awesome.
Their blog often features odd and nifty mp3s and such. http://blog.wfmu.org
Check out Bryce's blog – highly recommended (and not just because he played my shit a couple of times). Diane Kamakazie's portion of that site is also highly recommended (and not just because she does really cool tattoos!).