BillCue is a project mainly written in RUBY and JAVASCRIPT, it's free.
BillCue is a rails 3 app to help track events but developed with bill reminders in mind.
BillCue is a calendar app done in rails 3. It uses devise for authentication, devise_invitable to allow invitations, haml for templating, and rspec/autotest for unit tests.
It will first support email reminders but should quickly integrate sms reminders using Twilio and then expand again to support tweets and DM's.