If you haven't, you're missing out on something spectacular!
I recently stumbled upon one of their videos on YouTube. A friend wanted to show me this funny video, and somehow it turned into an afternoon of videos that simply make you smile and feel good.
Improv Everywhere was formed by Charlie Todd in August of 2001. According to Todd, a comedian who acts and teaches long-form improv comedy, Improv Everywhere is a group that pulls off huge pranks at the streets of NY, while documenting it all. On their website, they simply write "we cause scenes", and indeed, since they began "pranking" they have completed 85 missions, or more accurately, caused 85 scenes.
Be sure to check them out, and if you are one of the lucky residents of NYC, be sure to look for their pranks, for they might be just around the corner :)
Here are a few of my favorites:
Frozen Grand Central
There are many many more, so just go find the rest yourselves ;)