I have a Moleskine notebook that is my main spot for jotting down ideas. It accompanies me to ball games and auto repair shops, meetings and movies, restaurants and lobbies.
You know what happens to good ideas if you don't write them down. No matter how brilliant they may be - how certain you are that they will be carved into your memory - if they are not on paper they disappear.
The notebook heads off that problem but it also has another trick: It spurs other ideas. You see one thought that has become stale, sort of so-so, and yet now, just by reading it, you can think of another approach that has much more merit.
When it comes to creativity tools, pen and paper remain the Undisputed Champs.