Scribable: Great app that's perfect for those ideas that arise during meetings. I discovered this app from a reader recommendation and have to agree that I also find the "continuous notes" concept quite handy. $1.99
ShowMe: If collaboration is key and you'd love an easy way to explain what it is you're thinking while drawing, use this app. This app also has a flourishing community filled with educators using it to explain everything from grammar to algebra. Free
Paper: This is my favorite app, hands down, for sketching on my iPad. I love the way that the app organizes sketches into journals/notebooks and think that the drawing tools have a great natural feel to them. Did I mention that you can also share entire notebooks for great project collaboration? The app is free and comes with a black pen tool; additional drawing tools are available for in-app purchase. Free
Whiteboard: For a free whiteboard app on Android, look no further. While it does lack the community that ShowMe has, it is simple to use and built for easy sharing. Free
Adobe Ideas: While I mainly use this app for quick visualizations, you can use it for vector drawing as well. It's simple and easy to use and the app can really be as basic or complex as you need it to be. Also available for iPad. $9.99
What apps do you use to sketch an idea?