The folks at Tumblr have another equally simple and equally useful (IMHO) web application.

senduit.com

Here's what it does... It lets you share files privately without having to go through a lot of setup magic. You upload a file of up to 100MB to their site, set an expiration period (e.g., good for the next 30 minutes), and then give the web link to the upload file to whoever you want to get it. That's it. Simple and useful.