Paranoia is a re-implementation of acts_as_paranoid for Rails 3, using much, much, much less code. You would use either plugin / gem if you wished that when you called destroy on an Active Record object that it didn't actually destroy it, but just "hid" the record. Paranoia does this by setting a deleted_at field to the current time when you destroy a record, and hides it by scoping all queries on your model to only include records which do not have a deleted_at field.
Gemfile gem 'paranoia', '~> 2.0.3'
install gem install paranoia