I do ALOT of service calls and recently I have been doing alot of Whole House Surge Protectors. I buy them "DIRECT" pretty cheaply and sell them for a nice profit plus labor. The manufacturer I buy them from offers a $25,000 warantee with it. People eat 'em up(!), especially where the Utility Company charges to install them and then monthly, FOR LIFE!
I usually sell people on it when I'm already there for some other reason (serv. call) and I'd say maybe 50-60% of the time people bite. I believe they are a good product and it is a nice EXTRA at a service call. I'll usually install it right at the meter on the load side. No permit required. I cut the seal, snatch the meter out, loosen the connections, install the Surge Protector, wire it to the load side, tighten the connections, put the meter back in, close her up and that's that. Then we call the Utility and inform them that we've added a surge protector and they come and re-seal the meter. They have never required us to have a permit for this.
I'd definitely get into Surge Protectors, Whole House Generators, Burglar/Fire Alarm, Home Networking Centers, Etc.
There's a tidy sum to be had doing these types of specialty items.