What are some great OS X Dropbox uses?
1Password sync

Syncing 1Password's stored keychain across my Macs. I use 1Password for managing passwords and, more importantly, auto-generating random passwords so that I never use the same password twice. Dropbox makes this much easier. 

1Password's secure keychain file lives on my Dropbox, and each time a password is added or updated it's automatically sync'd across all my computers.