Ikea Hacker recently posted a great idea for us microbudget filmmakers. It's your basic rollerblade wheel dolly that rides on a prefab Ivar side unit from Ikea. While the dolly still needs to be constructed (see the comments for a parts list), the track is ready to go.
I love stuff like this. I might even build this version with a few changes. The L-brackets that hold the wheels make the platform ungainly and trickier to transport. I'd attach the vertical wheels right to the sides and the horizontal ones to the bottom. This would make it more portable and give it a lower profile.
Take note that this dolly isn't really for a camera op to ride on (though I guess it's possible for you lightweights), but for just the camera. Check out the link above for some very nice test footage using this cool DIY contraption.