These are my competitors. If you find one of them to be more useful than RegexPlanet, please let me know why!

Suggest a tool

Online

Expression Studio - JavaScript, but code examples for many languages

MyRegExp - Java (via Applet)

PHP Live Regex - PHP

PyRegex - Python

Pythex - Python

PythonRegex - Python

reFiddle - JavaScript, Ruby, .Net

Regex101 - JavaScript, PHP (PCRE), Python

RegexHero - .Net (Silverlight plug-in required)

Regex Online - JavaScript

RegexPal - JavaScript

RegExr - Flash

Regex Tester - JavaScript, PHP

Regexy - JavaScript

re-try - Python

reWork - JavaScript

RexV - Perl, PHP, Python, JavaScript

Rubular - Ruby

Fat Client

Expresso - Microsoft Windows/.NET

kiki-re - Microsoft Windows (probably others that wxPython supports)

Patterns - Apple MacOS

RegexBuddy - Microsoft Windows

Regex Coach - Microsoft Windows

Regexenator - Android

Regex Matcher - Android