Java: Regex Evaluator
I didn't write any back-end code for this one; its evaluations rely totally on Java's regex engine (i.e., java.util.regex). I created a GUI evaluator for the many times that I need to write regexes for my code. I find that GUI offers me more flexibility for my purposes rather than plain old command line.
Though the regex engine is Java's, it is pretty safe to use this even for other languages, say Perl. Exercise caution, however, when using POSIX character classes: they vary from language to language.