Let's talk about... Screen captures. The only external software you absolutely need to accomplish this task is an image editing program to save your output.
Followers of the Flag:
Press the Print Screen button on your keyboard to copy a snapshot of the screen to your clipboard.
On some keyboards, this key may also be labeled "Prt Sc", or have "Sys Rq" on the bottom half of it.
Holding ALT down while pressing Print Screen will copy the current (active) window instead of the whole screen. Very useful if you often find yourself cropping out the rest of the desktop.
Open your favorite image-editing program.
Windows comes pre-installed with Microsoft Paint™. It's free and does the job. The Vista version allows you to crop photos.
Under the Edit menu, click Paste to make the image appear.
Save the image to disk.
Don't forget that in order for Fubar to accept the upload, you need to save it as a JPG, GIF or PNG.
Children of the Apple:
Here, multiple methods can be used depending on your needs. Each method below describes screen-capturing a different area of the screen.
Press Command-Shift-3 to automatically save a snapshot of your desktop as a PNG file on your desktop.
Pressing Command-Shift-4 will change your cursor into a set of crosshairs, allowing you to click and drag a box around the area you want to capture. This will also save the PNG file to your desktop.
If you press the spacebar immediately after this command, your cursor will turn into a camera. Hover it over any of your open windows to cause your Mac to screen grab just that window.
Holding Control along with the keyboard commands above will copy the image to the clipboard instead of immediately saving it to a file. You can then paste the image into your favorite image editing program.