I'm addicted to screen magnifiers, because they let me see tiny details that make a huge difference in the appearance of stuff on-screen. My latest favorite is ColorPic, from Iconico (which is an interesting, artsy site in its own right). It has a customizable magnifier, and shows the RGB components of the current pixel, and lets you grab colors (which I don't usually need to do, but it is cool). And: it's free.