Windows/Mac/Linux and Android: Mounting your Android phone to your computer to transfer files can be kind of a pain. Free Android app Awesome Drop lets you drag files from your computer onto your device over the web, no mounting necessary.
While you could always use Dropbox's Android client to share files between your devices, not everyone has a Dropbox account. Furthermore, it doesn't actually sync files to your device's hard drive, so it becomes a multi-step process. Awesome Drop is quick, painless, and works from any computer.
Just open up the Awesome Drop app on your phone and enter the PIN it gives you in Awesome Drop's web interface, accessible from any browser. The HTML5-powered web app will then give you a window to which you can drag and drop any files you want, and they'll automatically appear on your phone, ready for use. Check out the video for a demonstration.
Awesome Drop is a free download for Android devices, and works on most browsers.