Github: unfold commit history
Adds "unfold all changesets" buttons (hotkey: f) above/below Commit History pages at github, letting you browse the source changes without leaving the page. (Click a commit header again to re-fold it.) You can also fold or unfold individual commits by clicking on non-link parts of the commit. As a bonus, all named commits get their tag/branch names annotated in little bubbles on the right.
Plan B: inline footnotes
Lets you click footnotes in Earth Policy Institute's "Plan B" series of books to bring them to your eyes, rather than have you chase down the page to the reference, and back again afterward. Hypertext!