Not necessarily small labels but the subreddit "listentothis" has been my source of interesting music for a while now. Its aim is new / overlooked artists. There is a "radio" like station as well which plays each submission in turn.
Since Bandcamp does not have a "random song" feature, I built a page that will let you hear a randomly selected album. I have a scraper that runs nightly to pull newly added albums, although it's not perfect (e.g., I never clean up deleted albums, but that's hardly ever been a problem). I call it Randcamp: http://dx3x.com/randcamp/