Behold, the Timethrottle! Basically it's a greatly enhanced (IMO) version of the slo-mo mod. Rather than a simple toggle between normal speed and extreme slow-motion, I set up the throttle so that you can step through various levels of slo-mo and even halt gametime entirely, using just the scroll wheel on your mouse. The wheel steps up/down through these states:
- normal time
- 60% speed
- 40%
- 20%
- 10%
- time stopped
When time is stopped you can mouselook around, but you cannot shoot portal beams until time is running again at some speed.
Additionally I've made a macro that binds your third mouse button (on my mouse I click the wheel itself) to toggle between first-person and third-person views, which really helps when you're coming out of a portal at an odd angle and you need to get your bearings. This toggle works anytime (not affected by slo-mo or freeze).
I uploaded it in the Tools section, because it's actually really useful for debugging your puzzles or taking screenshots of your maps. I hope you guys will agree. Link here:
http://forums.thinking.withportals.com/dload.php?action=file&file_id=64
To really make it user-friendly, though, I think it needs a bit more feedback. I don't want to display anything on screen though because it would ruin it as a screenshot-taking helper tool, so I'm thinking it needs sounds.
I was thinking something subtle, like soft beeps that got deeper the slower time ran. Sound as part of the user interface rather than sound effects per se (which might get annoying after extended use).
Any sound gurus out there want to take a whack and making sounds to go with this?

Just email the dude and get permission, or something. . .