I remember a comment a driver made from a thread similar to this and always wish someone would say it to me: the rider was going up a hill fast and a driver yelled out, "That thing got a hemi?!"
As for me, I've gotten a few thumbs up and some "nice bikes" from teenage girls doing carwashes and such. The most enthusiastic drivers I met were while I was on tour in Colorado. One guy in particular stands out; he was wearing a huge sombrero, driving an old, beat up Honda and he beeped the horn multiple times (he was passing opposite me and the horn was quiet) and waved frantically at me. Cheered me up on a long, straight, flat 50 miles on day one.