There is no later. This is later.

There's DJMark and DJTomdog, of course.  They play great music we should hear but can't get on the radio.  Radio sucks, we know this. 

And I just discovered a new add-on for Firefox: the PagePlaylist.

(Basically, it looks at any page you are on and if it has mp3s or links to mp3s, it will stream them in a handy Flash-based overlay.  It's awesome.)

Now I'm on the hunt for the sites that have great mp3 links.
So far:
Lost in the 80s
Aurgasm
WFMU's Beware of the Blog (currently featuring an excerpt from the first audiobook release of Slaughterhouse-Five)
Headphone Sex
Culture Deluxe
Of course, 3hive
The always-interesting Music for Maniacs

And TofuHut has a directory of sites with mp3 links---it's where I found most of the above.

If you know of any other great mp3 sites, let me know.