I am a light sleeper and spent years sleeping with a fan on to block out the noise around me. After having babies, I needed something that wasn't as loud as a fan but still blocked out some background sounds to allow me to sleep.
The Marpac 980 Sound Screen and SleepMate Sound conditioner was a great find. It is small enough (about the six inches around, and four inches high and weighs only 2.5 pounds) that I have carried it on vacation with me.
It has several settings ranging from loud to soft (although I sometimes wish the softer one were a little quieter). The noise is pleasant and sounds like a fan, but the unit does not blow air. It helps me sleep at night, but I can still hear the babies if they wake up.
It is durable and has a long 8 foot long cord, which is a nice feature. Overall, I would highly recommend this white noise machine.
Update On Jan 23, 2011: I have owned this unit for nearly four years and have used it every day since then. It still works as well as the day I bought it. I've also since talked three family members into buying their own.