Software Drum Machine
I'm sure there are several software drum machines available, but I've found Jammer Pro very useful. This program is similar to Band in a Box in the fact that it can create an entire song and melody line from a simple chord progression. The way it works is pretty neat. I can select the style of rhythm I want and it will create unique drum patterns based on that particular style. You can let it create as many different variations and fills as you would like and then just keep the ones that fit best in your song. Then you can arrange the different rhythms to create your song.
There are also many parameters you can mess with to tell it how often you would like a snare to hit or other percussion. Using the adjustable parameters you can create a completely unique drum part that sounds like a live drummer.
I believe where this product beats its competition is the fact that the rhythms sound very natural. They don't sound like a computer playing a midi drum part. And, it's a fairly inexpensive peice of software. (About $50 bucks or so) You may even be able to find a trial version somewhere on the net just to see if it will work for you.
I used Jammer for the drums on my last CD and they came out great.
Good Luck