They definately can be used in freshwater, and IMO are better than canisters or any other filters you may use,because they can hold ALOT more filter media, heaters etc can be hidden int hem, instead of in the main tank, they give more water volume and they are easier to clean...
Sumps etc are used in saltwater ALOT more than freshwater, so thats why you find the most info about them on saltwater sites... There are people on this site with freshwater sumps, but for alot of people its just easier to use a canister... Marine people use sumps to hide equipment (alot more in marine than freshwater) and for extra water volume (very valuable in marine) and for extra rock and stuf (rock is the main filtration in a marine tank)
http://www.melevsreef.com , even though its marine, has alot about sumps etc, so might pay to look there...
HTH