Rufus-scheduler is a thread-based scheduler written in Ruby. It lets you write code like

[sourcecode language="ruby"]
s = Rufus::Scheduler.start_new

s.every '10m' do
  puts 'open the window for the cat'
end

s.at '2012-01-01 12:00' do
  puts 'reminder: wife's birthday'
end

s.cron '0 22 * * 1-5' do
  puts 'activate the security system'
end
[/sourcecode]