I have just taken delivery of some flash gels from http://www.flashgels.co.uk/ . The gels, as you can see from the photo come pre-cut and with the velcro you can order are pretty much ready to go as soon as you get them.
For those not in the know, flash gels are used to ‘counter’ the effects of certain light sources and as a result reduce the effects that it has on the final image. An example would be Fluorescent lights. These tend to leave a horrible green cast over an image, by placing a ‘window green gel’ over the flash head and turning the white balance setting on your digital camera to Fluorescent this colour cast can be removed. A similiar result can be achieved by placing a Colour Temperature Orange (CTO) gel over the flash, turning the white balance on the camera to Tungsten or Incandescent, this will reduce the effects of this ‘colour’ of light coming from Fluorescent lights.
Further information and examples can be found on http://www.strobist.blogspot.com/ and explains this in further detail and opens up many more possibilities when using your flashes off camera.