ESC: Pauses the game.
1: Weapon category 1. (crowbar, grav-gun and physgun)
2: Weapon category 2. (handguns)
3: Weapon category 3. (smgs and assault rifles)
4: Weapon category 4. (shotgun and crossbow)
5: Weapon category 5. (explosives)
6: Weapon category 6. (other)
Q: Opens spawnmenu.
W: Forward.
E: Interact. (buttons, doors, NPCs all that jazz.)
R: Reload. (only works if current weapon is missing one or more bullets.)
T: Spray an image.
Y: Chat.
U: Team chat. (useless.)
A: Strafe left.
S: Backwards.
D: Strafe right.
F: Turn on/off the flashlight.
TAB: Display the leaderboard.
SHIFT: Sprint.
Z: Undo recently spawned entities.
X: Voice chat.
C: Open the context menu.
V: Noclip.
B: Zoom. (cannot shoot while zoomed.)
CTRL: Crouch.
ALT: Walk slowly.
SPACE: Jump.

Note: This does not contain modded weapons.

Crowbar (left click to swing)
Gravity Gun (left click to punt objects or NPCs, right click to pick up an object.)
Physics Gun (left click to pick up an object, right click to freeze held object, any object that is frozen you can press R to unfreeze it all.)
Pistol (left click to shoot) Fun fact: The pistol is based on a real pistol called the USP Match.
.357 Magnum (left click to shoot)
SMG (left click to shoot, right click to launch SMG grenades) Fun fact: The SMG is based on a real SMG called the MP7.
Pulse-Rifle/AR2 (left click to shoot, right click to launch energy balls.)
Shotgun (left click to shoot one shot, right click to shoot two shots.) Fun fact: The shotgun is based on a real shotgun called the Spas-12
Crossbow (left click to shoot, right click to use scope)
Grenade (left click to throw, right click to drop gently)
RPG (left click to launch rocket)
Camera (left click to take screenshot, hold right click and move mouse to rotate and zoom in/out)
Tool Gun (controls entirely depend on what tool you have. but it will tell you at the top right of your screen.)

Extra GMod weapons.

Bugbait (left click to throw, right click to call antlions)
S.L.A.M (left click to throw, while standing in front of a wall left click to place it on a wall, whatever touches the laser detonates it, right click to detonate)
Stunstick (left click to swing)
Fists: (left click for left jab, right click for right jab)
Manhack Gun (left click to spawn manhack, right click to spawn rollermine)