If you’re on a computer that doesn’t have Dropbox installed, or you’ve just been emailed an attachment and want to quickly save it to your Dropbox, Send to Dropbox will get the job done for you.
Just create an account (it’s free, but you can donate if you like), and you will be given a special email address. Any emails with attachments that are sent to that email address will be uploaded automatically to an Apps/Attachments folder in Dropbox. You probably want to set up your emails to forward to that address so you don’t have to manually do it.
Send to Dropbox has some other cool features, too, like folder organization, message body copying, and email whitelists. It also expands zip files and manages folders for you.