Quake Done Quick. The original and still the best 
This.
I couldn't believe it when I saw it - it took me a long time to get through each level back then, and this guy just drops a grenade, takes a few steps back, runs, jumps just as the grenade explodes, and gets to the exit in a few seconds.

Things like the HL2 speedruns were amusing, until you found out that they were exploiting engine and map weaknesses, like bunnyhopping, and crossing a level just outside the playable area.
And some of the Portal speedruns were just downright depressing.