With some regularity, the guys at 1kM1kT have a competition to do just that. In fact, there is one now, as I write this during November 2013. You can win 30 sterling in vouchers to meet "your gaming habit needs", and all you have to do is lock yourself up for 24 hours somewhere and write that game.
If you think it's not possible - you may be surprised - I participated four times, and I was both proud and surprised of what I could do in such a span of time. It's quite a boost to see what concentration and time-limits can do for your creative brain. Later versions even became full fledged commercial games of which I'm proud - like the cooperative card RPG Ringworld Zombie.
Even when you don't feel like participating, or if you have no time to do so - you might want to look at the results of others' efforts, and be amazed with at least some of them.