EZ Drummer recently had a really great sale. If I didn't already own and use Addictive Drums I would definitely have jumped on the deal. As for me, AD works flawlessly in Reaper and I love using it.
If you need something really quick and dirty that's MIDI based, one of the users over on the Reaper forums created a little drum-sampler set of tracks. Search for ReaDrums and you should be able to find it. I've tried it and even used it to create rough beats for several songs. We used it to keep time when doing scratch tracks with my band one night. The drummer wasn't available and we didn't have the luxury of rescheduling...