Qtrax, which has signed all four major labels, EMI, SonyBMG, Universal Music Group and Warner Music Group, launched yesterday with 25 million songs ( compared to around 3 m for Amazon and 6 m for iTunes ).
The downloaded songs are not compatible with iPods and have to be played via a proprietary player. Ads are displayed during playback, even on music devices so it's not for everyone, but it is legal.
Right now the service is down from all the attention it is getting. This is what I got when I went to the site ( "Due to overwhelming demand, Qtrax.com is currently unavailable. Please check back in 24 hours to download the first, free, and legal P2P music application. Thank you for your understanding." )
Once the bugs are worked out this should be a good alternative to downloading songs from BitTorrent and thus breaking the law.
Go see Qtrax.com and see for yourself.
UPDATE: Read Labels deny deals on file sharing