I've used the extremely popular Whenever on projects that rely heavily on scheduled tasks, and it's great. It gives you a nice DSL to define your scheduled tasks instead of having to deal with crontab format. From the README:
Whenever is a Ruby gem that provides a clear syntax for writing and deploying cron jobs.
Example from the README:
every 3.hours do runner "MyModel.some_process" rake "my:rake:task" command "/usr/bin/my_great_command" end every 1.day, :at => '4:30 am' do runner "MyModel.task_to_run_at_four_thirty_in_the_morning" end