When you're trying to collect together a lot of files, it's a pain to everyone on the same page. If you're a Dropbox user, Balloon is a web app that lets you create a single link you can send to people so they can add files to your Dropbox without needing a Dropbox account themselves.
Essentially, when you create a link in Balloon, it gives people access to a folder where they can drop in files, but can't browse the rest of the contents. It's handy when you're collecting together a group of files from various people. It's similar to Dropbox's "file requests" feature, but it's a little simpler to use, allows you to set a password and promises to support more than Dropbox in the future.