Right Click on the Dropbox Icon in your system tray -> Preferences -> Advanced -> Selective Sync Settings. Works for me on mac, pc and linux. Haven't tried in on my mobile devices.

EDIT: You can only choose which folders to sync, not specific files so that may or may not be what you were looking for.

Heartily agree with this. I don't need my library of vintage drum machine samples at work, but would love to have my personal txt file wiki along for the ride.