SomaFm is an online radio network which currently consists of twenty four unique channels of non-mainstream music. SomaFM was around before XM (since 2000) and is a viable alternative to that format. If you have an internet connection, you can stream SomaFM.
They're based out of San Francisco and began operations in the South of Market area. Each channel focuses on a different subset of electronica and/or roots music: classic americana to downtempo to exotica to avant-garde-ish/free jazz to indie pop to synthpop to Goa to drone/ambient to straight IDM to dubstep to drone doom to spy music to deep house to earthy ambient to spacey ambient. To qualify the last two: SomaFM has a channel that consists of different ambient tracks layered underneath a live stream of the public safety radio channel for the SFPD and one that does the same thing with historic NASA transmissions.
There's more too: channels devoted to music made for burning man and one for South By Soutwest and a channel that's strictly covers. During the holidays there are two additional channels for "different" takes on traditional music for the winter season.
Basically, they've got every non-mainstream flavor of music covered, excepting gospel, heavy metal, hardcore punk and classical.
Currently only one channel is available as a weekly podcast.
As SomaFM founder and DJ Rusty Hodge says, "Many of the records in my collection are now out of print and extremely hard to find, so I hope you don't get too annoyed that you can't easily find them online." Even so, there are tracklistings consistently updated online so if you want to hear something again, it can be done. Tracklistings are consistently updated on their website on the fly.
SomaFM boasts a total of 8000 CDs and 150,000 digital tracks their primary music library—which does not include the six DJs' personal libraries which will also be used from time to time. It's fairly rare to hear the same old song or artist twice in one day here.
SomaFM operates 100% from listener donations. Check it out. If you like what you hear, consider donating.
11/18/12: just found out about Digitally Imported Radio
but haven't given it a listen yet. I probably won't. It's available for mobile devices apparently.