Windows/OS X/Android: Sharing files between different operating systems usually involves emailing them back and forth as attachments, but Any Send is a slick alternative that acts like a cross-platform AirDrop clone.
The concept and execution of Any Send is a lot like previously-mentionedInstashare, except it works with more platforms. Once you have the app installed on multiple devices on the same Wi-Fi network, just copy a file to the clipboard, or open it in the Android app, then click the name of the device you want to send it to. Within seconds, the file will appear on the target device. The concept is simple, but the execution is pretty much flawless.
Any Send currently supports Android, Windows, and OS X, but iOS apps are in the works as well. Obviously, there are lots of methods to move files between devices, but for just getting one file where it needs to go, Any Send is as fast a solution as you'll find.