I know there are people who have computer fans direct wired into the wall and they've worked fine. This does not change the fact that it is a HUGE fire hazard and an accident just waiting to happen. I'd really hate for someone to lose their clutches(or house) because they didn't wire their electrical properly. If you have an old cell phone charger(or any sort of wall charger) hanging around you likely have a transformer that will work for you and you won't have to spend ANY money other than on the fan itself. Also, you guys spending $20+ on a fan are nuts. You are throwing your hard earned money down the toilet.

For example:

This 120mm(appx 4") fan is $4(80mm fans go for $1 on that site and work fine if you need smaller) and will be big enough for most incubators.

Its power draw is:

Operation Voltage: 6.0~13.8 VDC
Rated Current : 0.16 A

Plugging it directly into a 120 VAC source is GOING to eventually burn it up or short something out. Using a phone charger you have lying around is free, safe, and effective PROVIDED the charger is within the limits of the fan. You can check on the charger itself what it is rated at. If it still isn't right it is very simple to wire in some resistors or whatever you need to make it right.

Seriously, don't risk an electrical fire to save a few minutes on something so simple...