FilePhile is like an IM client, except it's for files. It uses an advanced and cryptographically secure protocol that can transfer unlimited files of unlimited size directly between computers, even if one or both users is behind a firewall!
It can also recover from pauses, restarts, crashes, or even changes in external network address. This means that you can start a transfer at home, close your laptop, go to a coffee shop, and when you open your laptop and go online the transfer will simply continue.
The basic FilePhile client is free, and a Pro version is on the way. Runs natively on Mac, Windows, and Linux/Unix, meaning that you can use it to transfer files with anyone regardless of what operating system or computer type they are using.