Wired Marker is a Firefox extension that lets you highlight sections of Web pages for future reference. Indeed, it's akin to running a highlighter over passages in a book, but with a lot more power.
The extension adds a Wired Marker sidebar folder to Firefox. After "highlighting" text, pictures, tables, and/or selected portions of the screen (which you can do in one of several colors), you drag and drop that selection into a bookmark folder. Meanwhile, Wired Marker remembers and displays those highlighted areas on future visits to that site (whether you bookmark it or not).
Personally, I'm loving this without even touching the bookmark folder. It's really handy to see highlighted areas when you revisit a Web page, as it saves you having to scan for the area(s) you found important in the first place. [via jkOnTheRun]