I have accounts on most cloud storage services (Dropbox, Box, GDrive, SkyDrive, SugarSync and Cubby), but Dropbox is the only one I use, and I use it all the time.
I mainly use it to share files with friends, clients, etc. Another way I use it is to test html/css sites since its public folder supports hosting simple websites using just html/css/js (nothing server side). I also use it to automate things using services like IFTTT, Dropbox Automater and MyBackupBox.
There are probably others ways I use it that I can't think of at the moment. >.>