Source Physics Engine
The Source engine is a lot less flexible than other game engines
such as Unity or Unreal, however, it's focus on rigid body and
ragdoll physics has made it the foundation of some of the most
popular PC game franchises today such as Half Life, Team
Fortress, Counter Strike, and Portal. Source doesn't really do
anything fancy in terms of the physics engine, and all of the
previously listed games can be broken down to the player
exploring a stable environment, and interacting only generally
with other rigid objects. So it may come as no surprise that the
most advanced "moving parts" that source supports are gears and
pulleys. Where it strives, however, is in calculating the
kinematics of rigid objects.
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In the popular franchise Portal, the defining part of the game mechanics was the players ability to place portals through which she (as the player's character is a female) could then walk through and thereby travel past otherwise unmountable obstacles. The game was then taken one step further to where the player could place a portal on the ground below her, and on a wall above her (as shown above) and then fling herself accross large gaps through conservation of momentum through the portals.