From a Mac OS X terminal window, you can use:

mate myfile.rb

to open a given file in TextMate.

You can also ask TextMate to open all the files and folders in the current directory as a project, so all files are accessible from the project drawer:

mate *

Useful functionality from MacroMates (developer of TextMate); one of those small features that makes you go "ahhh...that's nice".