Who's Playin is by far the largest collaborative project I have worked on to date.
We created a webapp to dynamically find upcoming local artists based on a user's geolocation.
It utilizes Spring MVC to it's fullest extent, a lot of lines of React, required complex AJAX promises from 4 APIs, and we wanted it to be pretty.
Created in 2 weeks.
This was my first webapp with backend collaboration.
We asked the question, "What do I do when I want to play Fallout, but don't know what type of character I want to be?"
This webapp was the answer.