Firefox only (Windows/Mac/Linux): Firefox extension Wired-Marker permanently highlights text on web pages. Unlike web-based highlighters or offline options like previously-mentioned Scrapbook, Wired-Highligher is a hybrid of online and offline highlighting. So while you can’t send highlighted content to someone else or view your content offline, you can see your highlighted content automatically any time you browse to the original web page. You can highlight content in a variety of colours and customise what your highlights mean to better categorize your highlighted content. Syncing capabilities would be killer, but as-is Wired-Highlighter is a great extension for highlighter junkies who feel left out in their browser.