I just realized you can let anyone upload to your Amazon S3 account with a simple HTML POST form. There's a guide on up on the official AWS site... from 2008, so I'm obviously a bit behind with this it appears. But then again Dave Winer just wrote about sharing buckets so I guess AWS still has several relatively unknown gems.
For this purpose, S3 accepts uploads via specially-crafted and pre-authorized HTML POST forms. You can include these forms in any web page to allow your web site visitors to send you files using nothing more than a standard web browser.
The article is a good read. The process involves putting several AWS specific input fields in your form: ACL, successful upload redirect URL, AWS access key identifier, calculated signature and encoded policy document to authorize the form, and key (name the S3 object being uploaded).