Update: Reader Jay points out that certain Android phones, like some models of the Galaxy SII, have their own screenshot abilities baked in. On the SII, holding down the (soft) Home button and (physical) Power button at the same time should activate a screenshot, which then appears in your main image Gallery.
Install the Android SDK on a Windows/Mac/Linux computer
Installing the Android SDK isn’t that painful, really, but it’s not self-explanatory, either. You’ll need to have a Java runtime installed on your system--not a browser plug-in, but the full Java motor. You’ll also need to enable “USB Debugging” on your Android phone (usually in the “Applications” or “Development” section of your settings), and then make sure your computer can “see” your phone when it’s connected by USB. A few phones (including Samsung and Motorola) require their own special software or drivers to connect properly; generally, you’ll have an easier go of it on a Mac or Linux system (easy, at least, in the way you’ve come to expect to edit system files in Linux to make things work).
Once your SDK and USB drivers are installed and your phone is connected, you launch the “DDMS” program from inside the SDK. In that program, open the “Device” menu and choose “Screen capture.” It’s a window that shows exactly what’s on your phone, and you can refresh it, save a PNG of that screen to your hard drive, or simply copy the image to your clipboard.
Grab “No Root Screenshot It”
I have a special place in my heart for things that are named for exactly what they do. No Root Screenshot It requires you to connect once to your phone with a Windows or Mac system, but once you’ve done that, you’re able to take screenshots on your phone—until you restart it, at least. Then you have to connect your phone again and run the desktop application again. For most Android phones, that’s a fairly rare occurrence, though, and it’s still less painful than the always-connected SDK method.