Can work on almost any webcomic/manga page, preloads 5 or more pages ahead (or behind), navigates via ajax for instant-page-change, lets you use the keyboard, remembers your progress, and it's relatively easy to add new sites

Wait for something. Silently reloads the page every X seconds until some condition is met

Ignores whitespace on Github Diffs

shows new posts while reading a thread

Shows CSS-resized images (and some thumbnails) in their original size by holding Shift while hovering over the image (or links to images)

Autoloads the next page as you scroll, and uses the arrows to move between posts. Works for the whole Cheezburger network, IPB and vBulletin forums, and some other wordpressish pages