I've accumulated quite a lot of these, usually with purchases of other large format equipment. When I want to travel light, I can bring a set of 3x3" gels that I keep in one of these--

http://www.moleskine.com/catalogue/c...ts__pocket.php

And a little Voss 3x3" gel holder with barn doors, and it takes good deal deal of weight and bulk out of the bag compared to a wallet or two of glass filters, and for many of my older odd-sized lenses, the clip on filter holder is the most convenient method of attaching filters.

Lee thin polyester filters are also pretty good and generally more durable than gelatin filters.