The Ludum Dare 34 is this weekend!
http://ludumdare.com/compo/
Ludum Dare is an accelerated video game development competition where you make a game in a weekend. 48 hours for the compo and 72 hours for the jam.
I have tried this numerous times before and have had a lot of fun doing it. I have also learnt how to make games very fast and efficient.
As what Mike Acton said in the interview at Handmade Con (I watched the recorded stream as I could not attend being in UK), it is a great way to practice what you know and to learn against a (very) short time constraint.
In the 48 hour compo, you have to create everything from scratch in that time: code, art, sounds, music, etc.
Here are 2 of my (more successful) entries:
* Raycaster dungeon crawler -
http://ludumdare.com/compo/ludum-dare-33/?action=preview&uid=29029 - (This was practice with using Emscripten (C to asm.js compiler))
* Dodge the thing game -
http://ludumdare.com/compo/ludum-dare-28/?action=preview&uid=29029 - (The source is very OOP just because I was learning SFML at the time and thought I would just keep to the OOP style which I now regret. SFML is fine if you like OOP and is actually well programmed but quite restrictive if you don't like OOP)
I have done others but they either were too buggy or not complete.