Please share the link to ONE Ruby tutorial that you think deserves the title. If you have information about the intended target audience's skills, please include it in your answer as well. This will help readers choose an appropriate tutorial.
For beginners, it is Try Ruby! (in your browser)... and by leaps and bounds, i think.
There's always the poignant guide, although not recommended if you're in a hurry and/or hate cartoon foxes.
It's a bit outdated now, but the two Peepcode Rails episodes are very good, then I would recommend watching though some of the Railscasts tutorials to "update yourself" (they are very good, and cover smaller concepts, or plugins etc, so there are new ones every week or two)
Here is another one: Learn to Program! There is an online version of the 2004 book available.
I taught myself from the pickaxe book - the one online is the first edition but free, and physical copies are typically in bookstores whenever I look. The table of contents looks daunting, but running through the first few chapters gives you a pretty good overview of the language. The rest is either special case (Ruby and Tk, for example) or reference.