http://rubular.com/
Neat little javascript regex editor where one can write a regex and a target text and see it working in real time. Works for Ruby regexes but maybe not others.
https://github.com/varnish/varnish-devicedetect/
Simple regexes to handle device detection within varnish. Includes links to documentation and suggested implementations. Neat.
http://stackoverflow.com/questions/16828/how-do-you-do-a-case-insensitive-search-using-a-pattern-modifier-using-less
Enable case insensitive searching in less by type -i or launching less with the -i flag. Genius.
http://www.quinapalus.com/cgi-bin/match
A great little tool. Give it a vaguely regex like pattern and it'll give you a list of all the possible words. Want 5 letter words ending in me or be, gotcha! Great for domain name inspiration. Output is scripting friendly plain text. :-)
http://www.ilovejackdaniels.com/regular_expressions_cheat_sheet.png
Handy regular expression cheat sheet which explains all the major regex patterns.