There are too many Ruby sites to list here. We will only cover some of the more important ones. (In any case, there is an irony in producing a printed guide to an online medium. Always do a Web search for the latest information.)
This is the master site, the actual home page of the Ruby language. Source downloads, CVS access, and bug tracking access can all be found here. All the latest news can be obtained here, and this site may have the shortest average path to all other Ruby sites.
This is another important source of information for Ruby developers. Highlights include articles, links, and downloads (including the "one-click" Windows installer). Also found here is the full text of the book Programming Ruby by Thomas and Hunt (readable online as well as downloadable).
An excellent source of news and information, including a list of all open RCRs (Ruby Change Requests). For those who like wikis, there is an ever-expanding wiki here, containing (among other things) a Ruby Style Guide.
This very useful site holds the archives for the English ruby-talk mailing list.
The author's Web site, always a work in progress.
A good "front end" for Ruby news and information, often summarizing and encapsulating what is found elsewhere.
This site contains a number of Ruby "recipes" for various tasks. It is similar in spirit to this book, but it evolved completely independently.
The "Ruby Channel" created by Clemens Wyss. This includes a "live" version of the book Programming Ruby (with executable examples in the browser); it also includes a good tutorial and a Web-based Ruby interpreter. The content is available in English and German.
Guy Hurst's Ruby page: News, links, and more.
Another Ruby portal, this one in Japanese only.
A page with many useful scripts, brought to us by someone we know only as greentea.