Ask: Do You Use Software for API / Architecture Design?

I've spent the last 6 months doing a lot of high level Architecture and API design. Most of my thought gathering, brainstorm and sense making exercises have involved a whiteboard.

Whiteboards are awesome because you can draw and express ideas easily and freely, but the sketches are hard to save and catalog. I've tried UML and digital white board tools and frankly the input mechanisms really cumbersome and slow down the whole process.

Is there any tool out there that leverages the freedom of a whiteboard, while still remaining digital?