Screen Sieve is a nifty tool for Mac OS X that lets you search the contents of the currently active window at any time.
It highlights all occurrences of characters, words and phrases as you type. Screen Sieve works with any application that supports the Mac OS X Accessibility APIs. Applications that do include Mail, Safari, Xcode, the Finder and iTunes, as well as most other applications written in Cocoa and to some extent Carbon.
Requires Mac OS X 10.4 or later (Leopard supported), and is a Universal binary, so it runs natively on both PowerPC- and Intel-based Macs.