Reason comes with rex drum loops, akai mpc studio has midi drum loops. There's a site called loop cloud, and they were offering a 1gb of loops and 1.5gb samples when you start a free trail.

Bfd, superior drummer, slate drums all come stock with midi drum beats, and bfd and sd have expansions available.

Ive got all the stuff i mentioned here except the loopcloud stuff (haven't downloaded yet). The reason and akia are more electronic music based, the drum software more acoustic drum based.

What i like about having the midi is the ease of editing the patterns, and changing the sounds.

I haven't dug around but id be shocked if there wasn't an overwhelming amount of free loops available.

Actually, you can use drum loops (which are recordings of drum paterns for one or a few measures) OR you can use a Virtual instrument VSTi whichs use independant drum samples and control them in midi to create your own paterns.
Addictive Drums is a nice VSTi, you can buy different drum sounds package and even already made up midi packs..