Ever get to the BART station and see that you need to wait 20 minutes for your train? One great hack for this situation is to board a train going in the opposite direction since it speeds up perceived waiting time by a factor of two and it increases your chance of getting a seat. However, doing this can be risky in that you might go backwards too far and miss your train.
BART Backtracker is a web app that uses BART's real-time API to tell you how far you can backtrack without missing the train. It also knows which stations make for poor transfer stations (due to needing to navigate stairs) and has you avoid them.