A friend of mine showed me dropload.com today. Despite the possible scatological interpretations of the name, it's actually a nice little site. Very simply, you can use it to transfer files to a friend by uploading stuff to dropload via a web page and providing your friend's email address. They'll get an email with a link that lets them (and only them) download the file within seven days. They're only allowed to download exactly once.
Upload requires free registration, but download does not. And if you believe their privacy statement (I do) they won't share emails with anyone. Poking around the FAQ, the site seems legitimate enough.
I'll definitely be using this site again.