Free is always good. The University of California and Speech Privacy Systems each offer free white noise generators. The University of California's requires you to download the program and experiment with the controls. The Speech Privacy generator is online; you simply alter the controls on screen. These are not stand-alone machines, you must play the noise through the computer. However, being free, these programs give you an idea of what noise sounds like and how it can help you sleep.
Homedics manufactures a sound generator which retails for $9.99 as of 2011. This machine produces rain, ocean, white noise and babbling brook sounds. It also has a timer, so you can set the duration of the produced noise. It is battery operated, and is available in pink or blue trim.
The Pure White Noise Company sells a CD and a downloadable MP3 that duplicates white noise. The company claims many machines have just a little speaker, which cannot duplicate the full frequency spectrum of pure white noise. The company also states many machines use a sound loop, which can have annoying glitches every time the loop starts.
The Marpac Company manufactures a variety of white noise generators. The top of the line unit is the Marsona 1288A, which retails for $129.95 as of March 2011. This machine produces a variety of sounds, including white noise. The machine provides eight base sounds, which you can vary to develop combined sounds.