Komodo Edit is free and works pretty well. It needs more than a minimal configuration to run okay. On the Mac, a lot of people like Textmate. I find the 'e' texteditor just fine for a windows laptop -- it's cheap but it's not free. Komodo Edit pauses and goes to sleep (or garbage collects) on my half-gig laptop, I simply ran out of patience. Both 'e' and Komodo edit do just fine with Ruby and Rails. Komodo has more features. The full Komodo IDE is expensive, so I've skipped it.
The Aptana IDE is free but is a tremendous resource hog compared to the others. It has good functionality but goes to sleep for extended periods. Some people blame the Java/Eclipse underpinnings of Aptana for it's lack of responsiveness.
If your a really good emacs hacker you can make emacs do what you want but it seemed it was taking forever to configure and then it didn't work right for me. Last time I looked there are about three or four different packages you have to integrate into emacs to get it play as nice as the others.
I hear good things about vim. I've heard that NetBeans is good but a resource hog.
Visual Slickedit has a fantastic feature set but is definitely not free and not oriented toward Ruby and Rails, the last time I looked.