One I use all the time that is missing from the lists above: Agent Ransack.
By far the best search engine I've ever used, on any OS. Will search for strings inside virtually any file type I've tried (except for Outlook's horrible .pst files).
That includes exes and pbos. Got a vague error message in the RPT and you're not sure which addon is responsible? Just search the relevant pbos for the string in the RPT message.
Lots of search options including regular expressions. It is also very fast. Results can be copied to the clipboard as formatted text.
Oh, and it's free, with no spy- or adware.