(I see Haley beat me to it ) Others have found that these work reasonably well too, especially to keep temps lower than the room temp on hot days (~$100): http://www.thinkgeek.com/homeoffice/gear/6ad2/. One complaint about using this unit for incubation is that its temperature regulation can be fairly wide. You might find that if you set it to 74F, that it may vary as much as between 70F and 78F . It may be possible to stabilize the temps a bit tighter by adding a container of liquid to give the interior more thermal mass.
Let's hear from those who use it to find out more about temperature regulation issues .
I have a couple of http://www.thinkgeek.com/homeoffice/gear/6ad2/ incubators. As Dave mentioned, there is some fluctuation. I have them in a room that is around 80 degrees. One of the incubators is set at 68 and I get a fluctuation of 64 to 68. The other incubator is set at 80. The fluctuation on this one is 80 to 84.
Lance