The IGUS linear guide rail has been one of the most popular off-the-shelf Rails used for DIY camera sliders. It's a solid build and fairly smooth slide and has been the foundation for other companies to 'rebrand' back into the Video market. One of the 'cons' of this slider is the lack of exact speed control when pushing or pulling manually. There's some friction with this type of slider which could be a good thing, but also could be a bad thing trying to control a very consistent move across the entire rail.
This time Vimeo member weelian soh attaches a basic DC high torque motor and a PWM controller (to control speed) from eBay to a DIY IGUS rail slider to achieve a consistent speed all the way across. A very nice DIY if you don't mind the amount of noise a DC motor can produce, and If you want to find out more, there's a ton of photos and information about his build following the link in the details here: http://vimeo.com/21491431
If you want to start building your own slider, IGUS has made it easy by providing a pretty much ready to go kit of rails and carrier. You'll just need to mount it to your tripod and drill a hole to add a fluid head.