regexxer - regexxer is a nifty GUI search/replace tool featuring Perl-style regular expressions.
RegExr - RegExr.com is an online tool to learn, build, and test Regular Expressions.
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, ...
rubular - A ruby based regular expression editor
dnGREP - dnGrep allows you to search across files with easy-to-read results.
RegEx Generator - RegEx Generator is a simple-to-use application that comes with the brilliance of intuitive regex and is also helping you out to test the regex.