One rather annoying shortcoming of Windows Phone 7 is that you cannot take screenshots. While screenshot capability is indeed a feature that mostly targets power users instead of regular consumers, it does have the ability to spawn some interesting memes.
So, what do you do if you want to take a screenshot on your Windows Phone device, to show off its beautiful Metro interface? Well, there are two homebrew apps that can do that for you. Since they're not listed in the Marketplace, however, your phone must be developer unlocked to sideload the respective XAP file.
The first app, which is arguably the more straightforward approach, is ScreenCapturer v3. Just click on the link, and download and install the XAP. After that, start the app, tap "Start Capture Task", and then navigate to where you want to capture the screenshot. Press the camera button, and a screenshot will be saved to the Picture hub.
If you want to directly transfer the screenshot to your PC, without going through Zune and syncing it, you can use another app called WP Screenshot. As you'll see after clicking the link, it consists of two components: a desktop, and a phone application. All you have to do is to start both, and type in the local IP of your PC into the phone app (of course, both must be connected to the same network). To find the IP address on your PC, hit Win+R, type "cmd", hit enter, type "ipconfig", and note the "IPv4 address". Now, half-press the camera button whenever you want to take a screenshot, and it will be displayed in the PC application. From there, you can press CTRL+S to save it.
These two methods should work for most of you, except if you're running an HTC HD2 with Windows Phone 7. In that case, pressing or half-pressing the non-existent camera button is obviously impossible, and even if you've configured it to be tied to another button it won't correctly register. So, you'll have to download the ScreenCapturer v2, which works just like the newer v3 version, but sets a timer instead. Once the timer runs out, a screenshot will be saved to the Picture hub.
Of course, as all these solutions require multi-tasking, they'll only work in Windows Phone 7.5, known as Mango. Furthermore, you cannot capture screenshots in games or videos; a workaround there is to long-press the back button to bring up the application switcher, and capture the smaller thumbnail instead.