Here are some nifty tools. Their usage is not limited to testing though.
Bug Shooting. It’s another screen capture tool which has a more catchy name than what I normally use i.e., Gadwin PrintScreen. Its edge over Gadwin is that if you intend to make some edits (e.g., add some arrows, circles, or some text), it has those functions immediately available after taking the screenshot. Whereas in Gadwin, you’d still need to open some image editing tool like Paint. With Gadwin, your screenshots are automatically saved into your capture folder. With Bug Shooting, you’d still need to save the file manually. But alternatively, Bug Shooting has an option of automatically sending the screenshot to an application. E.g., after taking a screenshot, you can send it to your email application and the image gets automatically attached.
Pixie. This tool acts like a color picker. So while it’s running, you’d just have to point your mouse over any part of the screen and Pixie returns the color you’re pointing at in hex, rgb and other formats. It also gives out the x and y positions.
JR Screen Ruler. If I remember correctly, this one was shared by Tats a long time ago. It offers a better alternative to using your “man-calipers” (as Chry puts it), or resizing some other window to use as a make-shift ruler. With this you can measure the length or width of certain screen objects in pixels, inches or centimeters.