Mac: File sharing on a Mac is fairly straightforward, but that doesn't mean there isn't room to make it even easier than it already is. Infinit is an app that allows you to quickly send files to friends or coworkers regardless of whether their computer is on.
Infinit is something like an always-on AirDrop. Type in the recipient's name, and you can start sending them a file. Once they accept it, it will start the transfer. It can be paused, quit, or stopped at any time and resumed later.
In our tests it was faster than AirDrop on a local network and Dropbox online. More than anything, Infinit makes transferring files easy since you don't have to worry about anyone's connection dropping or cloud storage space filling up. The one caveat is that both people have to have the app up and running for it to work, but at least it's free.