What is "good" depends on what is most useful to you. For me, though, these are the key features for a good regex editor (besides the ability to test and create regular expressions, of course, which is a prerequisite to be called a "regex editor" :-) :
- Displays matches hierarchically with captured groups.
- Explains/analyzes an entered regex in plain English, showing a hierarchical tree.
- Translates your regex into code for a language of your choice.
RegexBuddy, as @Max mentioned, does all these but there is also a free alternative, Expresso that also does them very well. These two utilities are the only ones I have found with the crucial ability to explain a regex. Here is an example from RegexBuddy: