"People constantly ask me what the best prototyping or mockup tool is. My standard answer is “whichever tool you are most comfortable/experienced with.” But just to cover all the bases and make sure that I’m not missing out on the “silver bullet”, I decided to do a review of all the tools that I’m aware of. You could (and probably should) distinguish between pure mockup tools (e.g. “Pencil”) and tools for creating interactive prototypes (e.g. Adobe’s soon to be released product code named “Thermo”), but for the purposes of this matrix, I lumped them all together, and I noted which have interactive capabilities and which don’t.
1) Low fidelity mockups for idea exploration and communication: Balsamiq
2) Interactive prototypes to test and communicate interactions: Adobe Flex (Thermo may change my mind)
3) High fidelity drawing tool: Adobe Illustrator or OmniGraffle" (Continued via Usability News, Dexo Design, Russell Wilson) [Usability Resources]