Android: Touchscreen keyboards, or even miniature ones, are not necessarily the ideal surface for getting things done. A physical keyboard and computer are just simply faster for many tasks, but there are a number of things they can't do that a mobile phone or tablet can. AirDroid bridges the gap by allowing you to control your device from pretty much any modern web browser.
The setup takes about five seconds. All you do is go to a URL in your browser (as specified by the app) and enter a code. Once you do that, you're ready to do a pretty amazing number of things. I was really surprised by how well you can manage your device from a browser window -- and it works on virtually any operating system (Windows, OS X, and Linux). You can transfer files and edit any existing files on your device. If you hate typing text messages on your Android, AirDroid will let you send, receive, and view them from its browser-based interface.
It allows you to install/uninstall apps, view photos, manage your contacts, manage your music collection, and view videos. With some devices you can even control your Android's screen, take screenshots, or make a screencast. It's a remarkably comprehensive app, operates securely, and is a free download.