In many cities, the transit systems have GPS data on the vehicles to allow exact prediction of when trains and buses will arrive at stops. This is quite handy if you live near a transit line, and people are working on better mobile interfaces for them, but it's still a lot harder to use them at a remote location.
It would be nice to have a small internet appliance for shops, cafes and other hangouts that are short walks from transit stops. The appliance would be programmed with the typical walking time to the stop, and of course which stop to track. It would then display, on a small screen when a vehicle was coming, and how much time you had before you could walk easily, and then before you could run and make the train or bus.
Failing the live GPS data it could just work on schedules. It might make a low-key but audible noise as well. It need not have its own screen, if the place has a TV already it could do an overlay on that, though flat panel screens are now only about $100.
Some transit lines have placed expensive outdoor "next bus" signs on their stops and shelters for these systems, which is great, but in fact it might make more sense to put an appliance like this behind a local shop window, where it doesn't need to be outdoor rated, and pay the shopowner or local homeowner.
To turn this into a moneymaker, it could be combined with a system to sell transit tickets (presumably through the cash register.) This is a win for the transit system, since transit lines without controlled stations waste a lot of time as the driver collects change and tickets as people get on. People with a pre-paid, pre-timestamped ticket can get on quickly and don't need a transfer. This even works for systems with distance based pricing. I have often wondered why you don't see more selling of transit tickets at the shops around stops in order to save this delay. SF Muni went to "proof of purchase" instead of driver collected tickets so they could put ticket machines at busy stops to save the driver time, but they aren't everywhere.
For a cafe, it's a nice thing to do for customers, and even makes them more willing to stay, safe in the knowledge they can get their vehicle efficiently. A taxi-summoning function could also be added (press a button on the box to call a taxi) which could, in theory, also predict when the taxi will arrive since many of them have GPS networks now.