The first time I touch textmate is in my internship in Pyramyd. Seb, who is my chef and a very good and humorous french guy, recommends me to use it. Later, I find that I felt in love with this little app. It is so cool when using Mac to develop Ruby on Rails.
Got from wikipedia:
TextMate is a general-purpose GUI text editor for Mac OS X created by Allan Odgaard.Popular with programmers, some notable features include declarative customizations, tabs for open documents, recordable macros, folding sections and snippets, shell integration, and an extensible bundle system.
Now textmate has not only Mac version, but also the version in Windows and Linux. You could add the plugins in it to fit for your own needs!
Me, I use it to develop Ruby on Rails, it is possible to add some gems with textmate, so convenient!
Give u a very useful cheat sheet of textmate for Ruby on Rails :