I haven't been to the ferret forum for a while, but I'll try to answer your question PandaHamster.
There are many cat foods that are actually very good for ferrets. Don't take the food as simply cat food -- read the ingredients to get an idea of what the food actually is.
There are some cat foods that beat out the well-known brands of ferret food -- since ferrets have diets similar to those of kittens.
Both Iams and Marshall's produce very bad food for ferrets. They both contain corn as a third ingredient, which means it is a fairly high percentage of the kibble pellet itself. Ferrets are obligate carnivores, so their food should contain real, quality meats as the first 4 to 5 ingredients -- before vegetables or other fillers start appearing.
Contrary to the above post, Eukanuba kibble(both chicken & rice or lamb & rice)is a great food. It ranks very high in terms of ingredient quality and the protein/fat/fiber ratio. I feed my ferrets a mix of Eukanuba, Zupreem Ferret, and Innova EVO.
Innova EVO, by itself, is notorious for producing foul smelling feces. This is partially why I use a mix of foods, not to mention gaining the benefits that one food may have over another.
Overall, if you're looking for a good ferret food -- try to stay around the following guidelines:
Protein -> 33-45%
Fat --> 20-25%
Fiber --> Less than 3%
Also: Make sure that at least the first four ingredients of the food are meat. Example --
Eukanuba: Lamb, Beef Liver, Catfish, Chicken Meal
Innova Evo: Turkey, Chicken, Turkey Meal, Chicken Meal, Potatoes, Herring(potatoes are used as a binding agent for the kibble)
Other great foods are Natural Gold, Wysong Archetypal, Arden Grange Kitten, and The Ferret Store Ferret Food. (Source: Artroxy's Food Chart)
Good luck in finding a good food. I'd highly suggest mixing at least two high-quality foods. You'll see the benefits for both you and the ferrets in the long run.