iOS only: Your iPhone (or other iDevice) is a great tool for making movies and easily sharing them online, but when you're at an event you're not always the only one capturing great video. Flixlab is an app that lets everyone using it share video with each other so each person can edit together their take on a particular event.
Video sharing works automatically. You have to be in the same place, but once you are you'll soon receive a request to join an event with other friends shooting video in the area. When it comes time to edit your movie, you'll be able to pull not only from their clips but yours as well.
Flixlab does everything for you, so there are not very many editing options like with Apple's iMovie app, but what you lose in control you gain in ease of use. To edit your movie you just pick the clips you want, tag people in them, choose a transition effect, and then the music you want. You can use any music from your music library or choose a song from Flixlab's free options.
Everything works through Facebook, so when you're done you can share it on Facebook. There are good privacy options, too, so you can specify exactly who you want to share the movie with and avoid blasting it across your entire movie feed (if you don't want to).
Here's what a final product video can look like (if you only use two clips and are trying to make something dumb):
Flixlab [iTunes App Store]