There are two basic types, rollers and resistance trainers. Rollers are better for improving pedaling technique, so you probably want to go the resistance route. There are three types of resistance units: wind, magnetic, and fluid. I've heard complaints that magnetic, in particular, have "linear" resistance, i.e. the resistance doesn't increase proportional to speed. Wind is noisy. Fluid is quieter and supposedly gives the most realistic resistance. I have a fluid unit (same one as reiko) with adjustable resistance. The adjustable resistance really isn't necessary, you can vary resistance just fine by changing gears. If you're really into it, and have money to spend, some have fancy features, like Cycle-ops (I think), that will give power readings, pedaling analysis, etc.

FWIW, the Performance TravelTrac Millennium is a rebranded Elite
http://www.elite-it.com/jsp/c-Prodotti.jsp?ID=04%20f, though a little older model than is pictured on their web-site. I'm satisfied with mine. In fact, they're on sale right now ($150 vs. $200). (I don't work for Performance, just a satisfied customer.)

In any case, this is a good time of year to find a bargain in a trainer.

[edit]
Also, don't forget to set aside enough money for a big fan.
[/edit]