Press the key to the left of 1 - "`" on UK keyboards, "~" on US keyboards.

Console variables:
p.x: Player's x coordinate
p.y: Player's y coordinate
p.a: Player's angle (expressed in degrees)
r.floors: Render floors/ceilings
r.fps: Frames-per-second counter
r.objects: Render objects ("sprites")
To read a variable: just type the name of the variable.
To set a variable: type "set <varname> <value>": for example - "set p.a 43"