I want to create an application in Rails with plugin system. Potential user should be able to upload (or better install from repository) a plugin and install it enabling my application to do something more. This should be done WITHOUT FTP/SSH/any low-level access to server.
So are there any good gems or tutorials on how it should be done in Rails 3?
Have you looked at http://edgeguides.rubyonrails.org/plugins.html?
It doesn't seem to be 100% compatible with Rails 3 but it can get you started. Most of the plugin articles I've seen cover Rails 2.