This project rocks and uses MIT-LICENSE.
If you are developing Rails 3 app, let say some meeting planner and you can’t save records which have time overlap.
Add to your gemfile
gem 'validates_overlap'
In your model
without scope
validates :starts_at, :ends_at, :overlap => true
with scope
validates :starts_at, :ends_at, :overlap => {:scope => "user_id"}