Windows: Find and Replace searches multiple files on your system for a string of text and replaces it with whatever you choose. Think of it like the Find/Replace tool in your favourite text editor on steroids. The app is portable, supports regular expressions, case sensitive searching, and it's free.
If you're not interested in replacing text, you can just use "Find" to do a whole-system search for a string of text. For example, if you know you wrote a document about eucalyptus at some point but you have no idea where on your system it is or how deeply buried in the document your comments were, Find and Replace can find it -- whether you're on your computer or a work machine where you may not be able to install something like a desktop search tool.
The app is also capable of finding and replacing multi-line text, recursing directories, and even sports command line options. Hit the link below to download it, check out the source code or see all of its features in action.
fnr.exe - Find and Replace Tool [Codeplex]