The main site for Ruby is ruby-lang.org. You can download the source code for Ruby by clicking on the download link along the right side of the page.
The standard Ruby tutorial and reference, known as the Pickaxe book, is officially called Programming Ruby: The Pragmatic Programmer's Guide. The first version of the book is freely available on the Internet, at www.rubycentral.com/book. The second version is available on-line, either from bookstores or directly from the Pragmatic Programmers.
You also might want to subscribe to the Ruby mailing list—although I have found it to be too busy for my taste and get a daily summary via the automatic tools at Google Groups. If you want to subscribe to the list, you can do so via the Web at www.ruby-lang.org/en/20020104.html.