Yes they do! But be wary of them because they are highly toxic and deadly to you and your pets! There are many types of flea collar. Some of them are high frequency, gas based, absorption and etc. Here http://www.thebugsquad.com/fleas/flea-collar-on-pets . Just googled it. Also if you really want a flea collar for your pet, try to use the alternative route by choosing natural flea collars. If your pet is a cat, stay away from flea collars with “Permethrin”. If it's a dog, avoid "Tetrachlorvinphos". And make sure the flea collar is free of Propoxur as this has cancer properties, something you definitely want to avoid!
While flea collars may work, there have recently been instances where the chemicals in flea collars have made an animal sick or even caused death. Many people opt to get a flea treatment that is safer from their vet, such as Frontline brand.
Flea collars aren't really the best way to get rid of fleas for your beloved pet. The extent of protection, if they really do work, is only the front part of your animal. In the end, your pet will still have fleas.
Flea collars only really keep fleas away from the collar itself. If your cat or dog has fleas. you will want to bathe the animal as best you can — or have the vet do it — and apply something like Advantage to the animal's back and neck area. A flea collar can then help keep fleas away, but shouldn't really be necessary. Also, fleas can't live in some areas once they are killed, like in Northern Nevada, so your animal might only need a flea bath if you live in a place like this.