Jake is a cross-platform collaborative file-sharing client that lets you create a folder and keep it synced for everyone you invite. It's built on open-source tools like Jabber, and seems to have a lot of features going for it. You can't really beat it for ease of use: setup is just adding a folder and inviting people, with no server-side fiddling to do. Even better, you can set Jake up on your intranet if you want to use it for work-related purposes.

The main difference between Jake and similar solutions like Dropbox is that Jake doesn't store files on a central server in the cloud. It just uses Jabber, XMPP and other open-source tools to sync the folder for users when they're online. Although Jake saves a log of the changes users make to the folder, it doesn't back up old versions of the data - again, no central server. This makes it slightly less powerful than some other, similar file-sharing services, but it makes setup a lot easier.