dnGREP - dnGrep allows you to search across files with easy-to-read results.
PowerGREP - Quickly search through large numbers of files on your PC or network using powerful text patterns to find exactly the information you want. Search and replace with plain text or regular expressions to maintain web sites, source code, reports, ...
AstroGrep - AstroGrep is A Microsoft Windows GUI File Searching (grep) utility.
The Silver Searcher - A code searching tool similar to ack, with a focus on speed.
DocFetcher - DocFetcher is a portable German/English open source desktop search application.
SearchMyFiles - Alternative to the standard Search For Files And Folders module of Windows. Duplicates search is also supported.