This week's book giveaway is in the OCMJEA forum. We're giving away four copies of OCM Java EE 6 Enterprise Architect Exam Guide and have Paul Allen & Joseph Bambara on-line! See this thread for details.
I assume you mean what are the advantages of Ruby over some other programming language. I would say that Ruby thrives in terms of having a simple language (to read and understand) that still packs a serious punch. With Ruby you can do more with less in the sense that many times a given problem can be solved in far less ugliness with Ruby than with, say, Java or C#.
A lot of the "punch" is due to language features such as closures (and blocks) and meta-programming.