The difference between something merely existing and something remarkable existing. If you find that someone has already done your idea, and they've executed well, consider abandoning your idea. If they haven't solved the problem well enough then it doesn't matter that they exist.

One of the greatest sources of motivation in solving a problem is the anticipation to use the solution. If a good solution already exists then you're not going to have that.

Think of the Zune team. Those guys were trying to solve a problem that the iPod had already solved so well. Where's the excitement? They had to make up artificial problems for the Zune to solve (WiFi song sharing) just to get excited about it. The problem is that the market doesn't care about artificial problems.