HD Remake: http://youtu.be/bG7sfXitotA (with settings that work in the modern source engine)
I lowered the graphical quality of Half-Life 2 by a huge amount by creating a custom configuration file, and this is the result. I didn't disable vertical sync, so the whole time the game was running at a solid 60 FPS, and I have no idea how high it could've gone if I had removed the cap.
The settings used were the following. Put them into a config file to see the effects (note that this is for an older version of the source engine and likely won't produce the same effects as in this video anymore):
I compiled that list together from various videos I found, as well as from messing around with all of the commands beginning with "mat_". Some of the strange effects you see are probably easily removed with no performance impact, so if you find anything, comment about it.