If you've ever forgotten a USB cable and need a way to transfer data to your Android phone from your PC (or vice versa), Software Data Cable is a free app for Android that turns your phone into a networked storage device you can browse from your computer, no cables attached.
After installing Software Data Cable, the app allows you to connect to and browse your phone's files and folders from any computer on the same network. The app works best when your computer is on the same Wi-Fi network as your phone. Just type the IP address of your phone into your browser and you'll be able to see its contents, complete with the option to move, copy and delete files if you need to.
If all you want is to get a file from your desktop to your Android phone, you can already use Dropbox to do it quickly and there's nothing wrong with that -- it's faster than this. Software Data Cable is a better option for people who don't trust a cloud service with their data, don't subscribe to Dropbox or another file syncing service, or want the flexibility to manage their device from their computer.
It's free in the Android App Market.