If your talking about individual load selecting manual transfer panels, Gen-trans / Reliance Controls seem to own that market. These make a great option for running selected loads only in a typical house, particularly the ones with built in load meters, I have one a rebranded Reliance Controls panel at my house which does fine for my limited loads on a 3-5KW generator for now. Note it is often possible to find these sorts of panels for sale on ebay, craigslist, etc. for well below retail if you do a bit of digging and creative searching (generator panel, generator switch, transfer switch, double throw switch, etc.), part of the reason I suspect is they are often bundled with big box store standby generators, sometimes these generators are being installed in place of previous units, or with whole house switches, in off grid installations, which leaves the owners with a useless to them transfer switch.
Ike