Is there any GUI based debugger for Ruby? Just a debugger. I do not want a full IDE like NetBeans because they tend to get your project dirty with extra files.
In netbeans you can tell it to put the netbeans project files in a separate directory or you can easily ignore the nbproject directory with your project's vcs. Netbeans has by far the best integrated debugging I have seen and there are many other great reasons to give it a try. Don't worry about netbeans using a project folder. I highly doubt you'll be able to find a better free GUI debugger.
If the code completion stuff gets in your way with netbeans it is easy to turn off and only request code completion when you want it (ctrl+space). That was my biggest gripe with netbeans.
Check out Mr. Guid, which uses GTK+ and is cross-platform.
I haven't used it in about a year, but I liked Arachno Ruby