A car that drives itself. It's not that I don't enjoy driving, but it would really make my commute a lot more enjoyable or my 12-hour drive to our vacation a lot more fun. I could read, play guitar, play games with the other passengers (games to be limited only by your imagination), or in the case of my morning commute... sleep. In many ways, it would be like public transportation only it would be ubiquitous, clean, convenient, and everybody wouldn't be forced to stop just because one person wants to get off.
The downside to this system is that it would certainly lead to more people driving even further since it's less of a strain.