Ludum Dare is an online competition that happens every four months. Developers join in from all over the world in order to create a game from scratch in 48 hours, all by themselves. The game must follow a theme, decided previously by 5 rounds of community voting.
You can use any libraries and any tools, as long as they are easily available to others. But any assets (graphics, audio and music) must be created just for the event. The definitions are a bit fuzzy on this, but nevermind about that.
The last Ludum Dare had an incredibly amount of activity, resulting in almost 600 games total. This is a huge success in comparison to previous years.
I did attempt to make a game, but there was no time to finish it. While I failed the deadline by a mile, it was an interesting experience and I had a lot to learn from it.
Here's a video of some of it:
I also have an older version you can play online.
Since I couldn't make it in time for Ludum Dare, I decided to keep working on the idea anyway. Here's the experience so far.