There is a lot of good commercial stuff. I would recommend the following awesome plug-ins that come with a large amount of drum library loops (audio and/or MIDI)... Check our links page
for more info on these...
Drumcore 3 has real drummers and audio loops can be dragged directly into Metro. It also has MIDI data with samples of the original drummer's kits. It is very easy to get some great sounding drum tracks. It also loads rex files with its setup program.
XLN Audio's Addictive Drums does not have the audio functionality but has a massive amount of MIDI loops that can be directly dragged into Metro and really sound good.
Stylus RMX has a lot of loops and functionality that makes a full sounding drum track easy. It also loads rex files (indirectly) and has add-ons as well as supporting up to 8 (or 16 I can't remember) sounds in one instance.
iDrum is a good plug-in and has add on packs although the library is not organized that well.
Toontrack has some great drum plug-ins although I have not tried them in a while.
FXPansion has GURU which comes with a large sample library, audio + MIDI. They also make BFD which sounds good.
There are plenty of other plug-ins with good drum sounds and MIDI libraries that come with them, e.g. sampletank + esoundz drum libraries, kontakt and others.