Dopplr is social network for travelers. While it is possible to sync your travel schedules with people using other applications, no application that I know of has focused on and delivered the answer to the "when are we in the same city next?" question so well. The upside of Dopplr is that it does this VERY well and is perfect for people like me who usually meet my friends while traveling. The downside is that it's not that useful for people who don't travel or don't have any friends who travel.
The other great thing about Dopplr is that it was founded by, funded by and is being developed by some of my best friends..