Version:            (using KDE KDE 3.5.3)
Installed from:    Gentoo Packages

hypothetical scenario:
I fall asleep in the evening listening to music from my laptop. its battery gets low, so it hibernates. in the morning I grab it and rush off to school. as the lecture is starting, I open my laptop to take notes... it picks up where it left off: playing music. I scramble to enter my long, secure password correctly and shut off the music before the teacher kills me, and everyone else laughs.

this would be a lot less painful if the screensaver had a way to mute the sound card easily - either a button on the password dialog, or a keyboard shortcut. :)
I'm sure other people could think of a million cool things to do from the screensaver. but this one seems like it'd be the most useful.