Firefox only (Windows/Mac/Linux): Firefox extension Find In Tabs searches for text across all of your open Firefox tabs. It does so by adding a Find in Tabs button to the find bar that switches from the normal single-page search to a multi-tab search. When you search with Find In Tabs enabled, it provides as-you-type results with context and tab number. This one could come in handy if you're doing some heavy multi-tabbed research and you need to find something you had read but don't remember which page you saw it in, but what's coolest about Find In Tabs has little to do with the multi-tab search.
The best part is the highlighting and context Find In Tabs provides in the search results, which is a killer feature when you're searching for text in large web pages. Rather than skipping through all the matches with the Next/Previous buttons, to see the text surrounding the match, you get a great heads up of all the matches and their surrounding context. Find In Tabs is free, works wherever Firefox does.