Having some paper and a pen can certainly do wonders - just last week I was working on something involving a lot of trigonometry (collision detection in a game)

Having some paper and a pen can certainly do wonders - just last week I was working on something involving a lot of trigonometry (collision detection in a game), and being able to visualize and think about it on paper helped a lot. Quick visualizations are probably one of the best use-cases in general.