This is a Firefox add-on for regular expressions with color highlighting (including submatches) and helpers for creating expressions. Observe in the screenshot shown below how the color changes to yellow as soon as the Regex matches a valid US zip code.
This is a simple add-on for Google Chrome and is inspired by Regular Expressions Tester 3.0. You can save a regular expression and the tool also shows the number of matches, sub patterns etc. The only thing that I was not comfortable with was that this tool wanted access to my browser history to work properly. Now I am not sure why was that required. Nevertheless, here’s a screenshot of the results after testing my Regex.
I could not find any add-ons for Internet Explorer that can test Regular Expressions. If you know of any such add-on, let me know and I will add it to this list.