Intercom gives a 90-day trial through its current users, allowing them to invite other startups and companies that might find interesting or necessary a CRM solution like this. Public Intercoms trial length is just 30 days.
Fiverr, this last christmas, offering a special promotion that can be listed between activating current users and getting more revenue in exchange of a piece of this revenue.
OKCupid triggers an time-limited offer when you try to downgrade your plan.
KISSmetrics uses the logout screen in order to get attention of the user in another of their products, a mobile app.
Mailchimp does a similar thing too.