I recall watching a TV show a couple of years ago (History Channel?) which documented the history of airline maintenance practices. United was highlighted as the pioneering airline of "on the go" maintenance (I'm approximating the phrase), which meant that airplanes no longer had to be pulled out of service every so often for weeks at a time just to fix small issues. I believe they debuted this maintenance practice on the Caravelle aircraft.

Forty years later, is United's maintenance still industry-leading? Do they still come up with advanced techniques and sell their services to other airlines?

I drove past a building labeled "United Services" at SFO the other day, which implies they do, but I'm not sure of the scope and quantity of their external business.

FWIW, the recent show "A Week in the Life of American Airlines" documents their maintenance practices very nicely - and it appears that, at least on the MD-80 series, American is very highly respected.