AirDropper lets people put files into your Dropbox ... without signing up for Dropbox
If you'd prefer not to email the person, but would rather IM or Twitter-DM them the link, you can check a convenient "I don't want to send them an email" box, and the link will be sent to only you. AirDropper also recently added support for sending multiple files at once, which makes it about a million times more useful.
My only minor quibble -- and it's very minor -- is that AirDropper creates its own folder when you connect your Dropbox account with it, instead of letting you specify a target folder. I'm not sure if that's an intentional security measure in the Dropbox API, or a choice on AirDropper's end, but it would be cool to see that extra bit of control so you don't have to manually sort files later, especially if you have multiple clients uploading to your account.
All in all, I'm giving AirDropper a two-thumbs-up, five-stars, I'm-totally-using-this-myself rating.