LeafDrums - Highly recommended by a number of users. ($25)
Tom Hicks Drum Samples - Whatever sequencer you use, you'll need drum samples. These are excellent quality, and they're FREE! Thanks, Tom!
Plug-Ins: There are literally hundreds of plug-ins available for enhancing your tracks. Plug-ins are grouped into two types; Direct X and VST. Both types are supported by most recording software (as is VSTi, by the way). Some plug-ins don't play well with others. Some plug-ins don't play well, period! Here are some that do.
Here are some freebie plug-ins that are known to sound good and work well: