Ruby Script Plugins
SketchUp Plugins - list on SketchUp site.
SketchUp Sage - comprehensive listing.
In addition to the Ruby and plugin tutorials on this site, you can use Google's Code Search.
For most ruby script plugins, all you have to do is copy the related "scriptname.rb" file/s inside the "Plugins" folder under your SketchUp installation directory (by default, C:\Program Files\@Last Software\SketchUp X\Plugins\ for PC). SketchUp must be restarted afterwards to load the plugin. Although most of them will show up under the Plugins menu, some might place itself in other menus like Camera, Tools...and sometimes Context Menu according to its function. Ruby files can be edited with Notepad; there, you can find additional information about the script's author, usage, version history etc.
See Also What are Ruby Scripts and Plug-ins? over on the SketchUp Sage.