Press ~ to bring up the console, then you can type the following console commands.

Code	Effect
drawfps 0	hide fps
drawfps 1	show fps
drawhud 0	hide HUD
drawhud 1	show HUD
globaltimespeed 0.5	slow motion (half game speed)
globaltimespeed 1	normal game speed
globaltimespeed [number]	set game speed
god 0	disable god mode
god 1	god mode
health [number]	set health to [number]
invisible 1	invisibility
invisible	disable invisibility
mucha 0	disable flying
mucha 1	flying
wireframe 0	disable wireframe view
wireframe 1	wireframe view