Calvin

Calvin is an app that makes it super simple to organize and make plans with friends. With Calvin, people can skip the inbox and collaboratively plan on a time and date for an event without back and forth emails and text messages. Calvin integrates with iCal, Google Calendar and Facebook. You can even store events without times, dates, or invitees and add the details as soon as you're ready.

It's a calendar, to-do list, and messaging app all in one. And best of all, you can use Calvin with your friends whether they have it or not.