TextMate - the best editor around.
TextMate ? The Missing Editor for Mac OS X
Firefox together with the plugin Firebug; a must have for webdevelopers.
And of course a lot of Adobe applications.
Here are some guides to how you can setup a working Rails development environment on your Mac.
MySQL:
Hivelogic - Installing MySQL on Mac OS X
Rails:
Hivelogic - Installing Ruby, Rubygems, Rails, and Mongrel on Mac OS X 10.5 (Leopard)
Good luck.
