Dangerous q to ask here, you'll get 100 different answers from 50
different people ;)
I'll give you 3: Scite, Eclipse and NetBeans

For small things I use scite, which I believe can work in OSX, but
more or less it'll be equivalent to any ruby-aware code-highlighting
text editor that will launch the interpreter for you.  The downside is
it's really just a text editor.
Scite, of 10
ruby integration: 4
text highlighting: 10
multiple file support: 4
version control: 0
stability:8


I'm using eclipse+RDT at work, and am still using it for my projects
there, because it's already setup and works.  The downside of it is
that it seems to have issues with following the object model more than
one or two steps, and can't do things like refactor a class name
across all files in the project, or even a search and replace across
all files in the project.
eclipse+RDT, of 10
ruby integration: 7
text highlighting: 10
multiple file support: 8
version control: 5
stability:4


That said, I use NetBeans (6.x beta, milestone whatever) for all my
personal ruby coding that's IDE-worthy.  It was a HECK of a lot easier
to setup, and on reasonable hardware (even on my old 1 ghz laptop
linux) is no slower than eclipse.  I haven't used it as much, but so
far it's giving me a good feeling, and the ruby integration is much
tighter (esp with the intelisensish type things)
NetBeans 6 Beta, of 10
ruby integration: 9
text highlighting: 10
multiple file support: 9
version control: 8
stability: 8