I feel the need to weigh in on this one. Let me preface this by saying that, as everything I post on here, this is only my opinion...
Before you use a drum machine, you should know exactly what a drum machine is used for. A drum machine is a song writing tool. That's it. It should be used to see how drums will sound with your song and, possibly, to give your recording drummer a good idea of what you want him to play. A drum machine is
not, nor should it ever be, used as a replacement for a "real" drummer.
That being said, if you need a free option and you have a Mac, you should look no further than the included software instruments in Garageband, which was almost definitely included with your machine. I'm going to assume, however, that you are using a PC.
I'm not sure of any free options on PC that are worth using -- even at free. You may have to bite the bullet and spend some cash. The good news, though, is that you can get pretty good results with relatively little money.
If you're serious about using great sounds for your songwriting demos, you should seriously consider the EZDrummer line of software, located at
http://www.toontrack.com/products.asp#EZLine .
At $179, it isn't exactly cheap, but it certainly isn't expensive, either. It addition to the main software, there are expansion packs that cost $89 apiece and are sort of niche specific. The main kit is a generic pop/rock kit, but the expansions include kits tailored for jazz, funk, country, hip-hop/RnB/drum and bass, metal, and other genres. Each pack -- including the main software -- comes with a selection of MIDI grooves and fills that were "played in" by some great drummers (as opposed to programmed by hand on a piano roll or similar method) including John "Jabo" Starks, Clyde Stubblefield, and Roy "Futureman" Wooten. There are also extra MIDI packs available for $29 each. All you have to do is fire up your sequencer, load up an instance of EZ Drummer and drag and drop your MIDI files. If you need a sequencer, check out Cockos REAPER (google it). It's free to try for as long as you like with no feature lock-outs. If you want to purchase a license, I think it's only $40 or so for a personal (non-commercial) license.
My advice to you is to get EZ Drummer to use as a song writing tool (either save up for it or hit up the parents, depending upon your age :-P...birthday, perhaps?). When you're ready to record a proper demo please, please, please use a live drummer. Save up whatever you can to pay for a decent studio drummer or find someone who will do it for free, just to get some experience and recording credits. Craigslist is your friend.
It's worth mentioning that I'm not an official endorser of Toontrack, I've just used EZ Drummer and Superior Drummer 2.0 for years and I'm completely in love with them.
Good luck with your songwriting, and let keep us posted on your progress!