Powerpoint. Not really a prototyping tool per se but it's super easy to draw shapes & text in it and almost everyone has it installed. You can duplicate and mod

Powerpoint. Not really a prototyping tool per se but it's super easy to draw shapes & text in it and almost everyone has it installed. You can duplicate and modify slides to simulate different UX flows, like going through a wizard or filling out a form. With Office 365 you can also easily share and collaborate on prototypes. The only thing that most prototyping tools add (IMHO) is better organization, more rich interaction and some premade graphics, but if you don't spend all day prototyping I think Powerpoint (or OpenOffice) is just fine. Task management / feature planning shouldn't be done in the prototyping tool IMO. Other than that: Pen and paper.