Photoshop and Illustrator are my second homes. Once pen and paper (and a little wireframing) have nailed down the requirements it's off to photoshop to really get into the details.
While working in MicksGarage.com Invision became an absolutely essential tool for me on a daily basis. It was already a central part of my workflow before the updates targeted specifically at mobile design were rolled out - which really put the cherry on top.
The collaborative aspects of Invision and the ability of stakeholders to comment / discuss the prototypes in the app were very valuable on both large scale, site wide projects and small changes to functionality.
Recent experience designing for responsive sites led me toFroont and some other design / prototyping tools made specifically for the complexities of responsive design.