A damn useful cross-platform file syncing service that works the way iDisk was supposed to.
Obscure Bits of Lore
If you lose Finder icon overlays and context menus, quit Dropbox and do this:
sudo rm -rf /Library/DropboxHelperTools killall Finder
…and restart Dropbox. It should ask for an administrator password to reinstall the Finder plugin, and all should be well.