I have been a fan of last.fm for something like four years now and love having all my music tracked/compared w/ friends and such. However lately I have been using pandora since i discovered you can have stations based off of multiple artists and songs, this discovery has kept me w/ pandora. Slacker's music selection seemed pretty sad to me, never gave me what I wanted.
As for music on my phone I used to use meridian, I really liked it, most the time, however I did not at all like the new UI brought about a month or so ago so when my droid got replaced a couple weeks ago I just stuck with the stock player. It's not everything I would wish for in a media player, but it is simple and works well. Currently looking to try some of the ones suggested here..