I'm thinking about using Ruby to write an installation script, but I'm not sure if all major versions of Mac OS X install it by default.
Which version(s) of Mac OS X have the Ruby interpreter installed by default?
The Ruby download page indicates that Tiger (10.4) and Leopard (10.5) both include Ruby. I can verify from machines in my own home that Lion (10.7), Mountain Lion (10.8), and Mavericks (10.9) all come with Ruby. I can't personally verify that Snow Leopard (10.6) contained Ruby, but it would be shocking if it was present in the previous and subsequent versions but not Snow Leopard.
If your target audience is people who bought their computers or upgraded their operating system on or after April 29, 2005, you should be safe using Ruby in your installer.