Buttonbeats.com is the best sample site I have seen They are not downloadable, but you can play them right on the site. really good stuff.
Buttonbeats is a good site, it is nicely presented but filled with Flash, so I am unsure of actually how to download stuff. Some of the samples and loops are actually in a poor grade format. I think it is set up for you to play samples "on the fly" however it would be nice to see how long the samples are or at least view the sample so you know when a kick would turn up.