FedEx Feeders- Mountain Air Cargo(SD3, Caravan, F27), Empire Aviation(Caravan, F27), Barron Aviation(Caravan), Wiggins(Caravan), CSA Air(Caravan), Corporate Air, West Air
UPS Feeders- Freight Runners Express(BE99, 402) , Superior Aviation(Metro, Caravan, 402), Air Cargo Carriers(SD3), Wiggins(BE99), Ameriflight(BE99, BE1900), Alpine Air(BE99, BE1900), Air Now(Caravan, EMB-110), Mid Atlantic(Caravan), PlaneMasters(Caravan), MartinAire(Caravan, Metro)
There are many other feeders for FedEx, UPS, DHL, and ABEX as well as companies who do just ondemand freight. The difference between FedEx and the rest is that FedEx owns all the aircraft that are operated on its feeder routes except the Shorts and maybe some of the F27's, where the rest of the feeders are owned by the companies who operate them and are on a contract with UPS, DHL, ABEX, ect. If you are looking for freight operators in a specific region of the US let me know and I may be able to help you out.