We have a bit "extended" collection containing some other sites than the two major ones Remus linked in here.
Have a look at the FAQ at Smustard to see more in the details how rubies work and how to use them.
Note that some scripts go to the Tools folder rather than thje Plugins folder. Since ruby scripts are basically text files, the authors usually put a few lines of information about how to use the script (and where to install) on the top.