Keynote is a piece of software from Apple and about a year ago has been thoroughly overhauled. You can download it for free:

What makes Keynote so great is the ability for quick, smooth and dirty prototyping, there has been plenty of articles written about this:

I’ve always been a huge advocate of using Keynote for basic prototyping because there are a lot of great animation/drawing goodies hidden in this app and it’s so easy to set a scene up. Same thing goes with motion graphics. Even if you’re not planning on making your final animation in Keynote, it’s an incredibly fast way to audition different effects and narratives.