The Skycaps outside of terminal 2 in Oakland (Southwest gates) must have a gold mine. There is usually a line to use them -- sometimes it's about as long as the one inside for Southwest baggage check in.

I'd guess about 40 bags per hour, with an average tip of $1.25 per bag. I'm also guessing that not every one of those greenbacks is declared on their 1040s. They might take home more pay than some of the pilots.