To view the coverage of your plugins using Rcov, first install the rcov gem with sudo gem install rcov, then copy and paste the following onto the end of the Rakefile inside your plugin directory:

require 'rcov/rcovtask'
 
desc 'Measures test coverage using rcov'
namespace :rcov do
  desc 'Output unit test coverage of plugin.'
  Rcov::RcovTask.new(:unit) do |rcov|
    rcov.pattern    = 'test/unit/**/*_test.rb'
    rcov.output_dir = 'rcov'
    rcov.verbose    = true
  end
 
  desc 'Output functional test coverage of plugin.'
  Rcov::RcovTask.new(:functional) do |rcov|
    rcov.pattern    = 'test/functional/**/*_test.rb'
    rcov.output_dir = 'rcov'
    rcov.verbose    = true
  end
end

You can now simply run rake rcov from inside your plugin directory which will generate an rcov directory with the results. Open rcov/index.html (if you are on OSX this will open automatically) in a browser to view the results.

Thanks to Mike Clark for his Rcov rake task for Rails which this task is based on.

Update (11-11-08): Changed code to use the RcovTask class.