Locomotive is a flexible one-click solution to Ruby on Rails development for Mac OS X 10.3+. In one self-contained application, it gives you a fully functional Rails development platform including: (but not limited to)

• The framework: Ruby on Rails
• The favored webserver: lighttpd with FastCGI
• An embedded database: SQLite

Locomotive includes the Ruby MySQL and PostgreSQL bindings. If you have MySQL/PostgreSQL installed, or want to use them, Locomotive is ready.

Locomotive will be nice to your existing rails installation as Locomotive is entirely self-contained.