There is an empty canvas.
You may place a tile upon it, but you must wait to place another.
Individually you can create something.
Together you can create something more.
-r/place Reddit Announcement 
r/place (pronounced "ar slash place" or simply just "place") is a collaborative art project created by the community of the website Reddit.com in April of 2017. r/place was a grid of one million pixels (1000×1000) that was initially completely white. Anyone with a reddit account could visit http://www.reddit.com/r/place between the first and third of April of 2017. Once there, users could pick a color, and place a single pixel of the chosen color anywhere on the grid, including pixels that other users had already filled in. Users were allowed to place additional pixels five to twenty minutes after each pixel they placed. r/place was hosted at http://www.reddit.com/r/place, and while the page still exists, users can no longer participate in the project and instead the subreddit is now dedicated to people simply talking about r/place.
r/place was created by the admins and moderators of Reddit as their yearly april fools joke. These april fools jokes are usually social experiments that involve as much of the Reddit community as possible, one of the most notible before r/place was r/thebutton in which there was a page with a button and a countdown timer, if any user pushed the button, the countdown would reset and that user would recieve a flair next to their username that showed how much time was left on the timer when they pushed the button. Nobody knew what would happen when the countdown reached 0, all that was known was that everyone could push the button once and doing so would reset the countdown for literally everyone. The result of this was that communities formed around the button, some people took pride in never pushing the button, whereas others took pride in pushing the button when the countdown was as close to as 0 as possible.
r/place also resulted in communities forming within the project as well, but to a much more notible degree than r/the button. This is because of the fact that in order to create an image on r/place an entire community was completely necessary. Since each user could only place one pixel every five to twenty minutes, most of the individual images on r/place were contributed to by hundreds of users, not to mention the fact that many images had to be redrawn or "defended" since users could place their pixels over pixels that others had already placed. Many of the largest images in place were created by existing communities on reddit, for example the big red box with black text in the center-top of the canvas is a Star Wars quote created by r/PrequelMemes, a community dedicated to making memes of the star wars prequel trilogy. Some communities also formed with the sole purpose of creating something on r/place, such as r/MonaLisaClan who wanted a recreation of the Mona Lisa in r/place and /rTheBlueCorner, a community dedicated to making the bottom right corner of the canvas as blue as possible.
The final image is a chaotic, detailed, and intriguing work of net art, but I think part of what made r/place so unique was that it was constantly changing, as there are tons of images that made their way onto r/place that did not end up in the final state of the canvas. Thankfully, there are plenty of timelapses showing exactly how this jumbled chaotic image emerged from giving over a million people the ability to add to a single canvas one pixel at a time.