Help with Custom Rest Actions

How can I create a method like show and link to it from index?

what should I put in routes.rb?
what should I put to link to it?

why do you want to link to it from index? do you mean an actual
"link_to" hyperlink? or do you mean that you want it to display INSTEAD
of index?

if the latter, in your routes.rb file put:

map.connect '', :controller => 'site', :action => 'show'

if your site were at www.mysite.com and your user tried to go there they
would be redirected to the show action of the site controller....