I only do original music, so, now that I think about that, as far as cover songs go... hmmm... you might have to check each site's rules. Because some of them, you must be the copyright holder. Anyway, for original music (and maybe covers is okay, not sure)...
http://www.soundclick.com
Possibly the largest but from time to time, their server is down. Unlimited uploads for free, I think. But if always being functional is priority, not acceptable. Hugely popular but their server issues are not good, IMO.
http://www.songramp.com
Three songs at any given time for free.

Others I have heard of but never used.
http://www.myspace.com http://www.broadjam.com http://www.garageband.com/ http://www.madeinmusic.com/ http://www.nowhereradio.com/ http://www.besonic.com/ http://www.exFolk.com http://www.ampcast.com/

Obviously, best solution is your own website, especially if you want to minimize downtime problems ease of use by a visitor.


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Me Bill
Yamaha PSR2000