My life revolves around PowerPoint slides at the moment. Long Zheng has posted a really cool slide deck demonstrating how you can use PowerPoint as a low fidelity prototyping tool.

PowerPoint Prototyping Toolkit (release 0.2) - istartedsomething

My thoughts? it's almost too good! Showing people a prototype of this quality could give the (false) impression that the software is nearly ready when you may still be in early stages/iterations. Dirty it up a bit by using a tablet PC to draw all the screens - then link them all together for that grungy, back-of-napkin design (or story-boarding) experience.