I'd be real careful about putting lights in the floor of a garage. Besides moisture (and snow and ice if you live in the north part of the country) there's gas, oil and all kinds of other flammable fluids that will get on the floor, and no matter how well a fixture is sealed there's always the possibility something will leak in. Also, you have to worry about explosive fumes that collect at floor level; you don't want an accidental spark blowing the place sky high.
There are LED and fiber optic lamps that would be a lot safer than incandescent or fluorescent lamps; do a google search on "driveway flush luminaires" and you'll get some hits, or go to this link and scroll about halfway down the page:
http://www.thelightingcenter.com/products/40/
Here's another possibility: go to
http://www.hadcolighting.com/, click on Products and scroll down to Inground fixtures. Low voltage rope lighting that's rated for outdoor use may be something to consider, too.
There are probably other light fixtures that would work but I can't give you any specific suggestions off the top of my head. I'm an electrical engineer so I know a little about this stuff, but there are so many different brands and styles of lights it's hard to keep up on everything that's available unless you're a specialist (at the office we have a shelf eight feet tall by twelve feet long filled with nothing but light fixture catalogs.) If there's a commercial lighting store in your area, you may want to stop by and see if they can help you out.
Again, be careful - a garage floor is a harsh environment and you don't want to use anything that's not specifically designed for this kind of application.