There was another topic on airports, which got me thinking about airport to airport connections for airports in the same region.
I remember being in China when we had to rebook for a flight the next day in Shanghai (were at Shanghai-Pudong) because the visibility conditions screwed up travel for hours and our flight was cancelled. Only they have two major airports, and we ended up staying at a nearby hotel. The next day we took a cab (pretty cheap) to the airport and took a special bus (airport bus #1) that went between Shanghai-Pudong and Shanghai-Hongqiao (which is a combination airport and major train station).
At least in the Bay Area it might be possible to take BART directly from SFO to OAK.
At least in my area, I think there might be some way to connect from San Jose Airport to SFO, although it would require a slew of transfers from the free VTA 10 line, to Caltrain, and then a quick BART connection from Millbrae. Before BART, there used to be a free shuttle between the Millbrae Caltrain station (which is now part of the Millbrae intermodal station) and the airport.
When I was in NYC, I wasn't sure if there was any easy way to connect between airports. I think there might be a bunch of private shuttle options to get between LAX and the other airports in the area, but public transportation won't be practical. Maybe Chicago? There are a few areas served by multiple airports like I suppose Dallas or Houston with the smaller airports that Southwest uses. Maybe around DC?