I'd try one of the many Napster free offers as a start off - the last one I used was a month free trial via the Independent newspaper.

I'm sure someone here would be able to tell you what to do with the downloads - ie whether you wanted to keep them or not ;). You can roam round Napster for quite a long time for nothing if you have a number of cards if you just want to listen - if you want to download and save a few gems use Audacity or something similar during your trial period/s. If you are going to want to use it on an ongoing basis then just pick one which has your sort of music, subscribe and keep going with the one that has most of your musical taste.

I guess it depends what it is you are trying to do and what sort of music.