All sorts of rear-hub variations:

Internal — Modifying the internals of the hub

• internal-gear hub
• coaster brake
• kick-back hub
• freecoaster

External — Modifying parts of the hub that aren't fully enclosed inside the hub

• flip-flop hub

Unattached — Modifying the rear part of the drivetrain, including things that aren't integrated with the hub

• retro-direct drive — one-offs only
• hybrid gearing = internal-gear hub + a deraileur

WholeDrivetrain — Modifying the whole drivetrain, but still involving the hub

• dinglespeed
• 3-speed internal-gear modified to use two-chains — only one ever made?
• two chains, with a motor driving the other side