A short-nosed truck (G1 Optimus Prime)
A different, possibly futuristic short-nosed truck (Powermaster Optimus Prime)
A definitely futuristic short-nosed truck (Star Convoy)
A long-nosed truck (Hero Optimus Prime)
A more differenter long-nosed truck (Laser Optimus Prime)
A gorilla (Beast Wars Optimus Primal and Cybertron Optimus Prime)
A different short-nosed truck (Machine Wars Optimus Prime)
A fire engine (Robots in Disguise Optimus Prime)
A (Cybertronian?) monster truck (Armada Optimus Prime)
A Cybertronian short-nosed truck (Energon Optimus Prime)
A Cybertronian/futuristic fire engine (Cybertron Optimus Prime)
Yet another short-nosed truck (Classics Optimus Prime)
A Peterbilt long-nosed truck (Movie Optimus Prime)
Another random fire engine-based thing (Animated Optimus Prime)
Mickey Mouse (Optimouse Prime)
A shoe (Sports Label Convoy)
Another long-nosed truck (Prime Optimus Prime)
Some sort of beast truck (Prime Beast Hunters Optimus Prime)
A pen (unreleased)
A bearbrick (unreleased)
Various Cybertronian somethings
A sports car (Alternity Optimus Prime)
ETA: A baseball cap (thanks liegeprime)
ETA: A DVD case (thanks BigTransformerTrev)
Absolutely nothing at all (various PVC and static action figures, Action Master Optimus Prime)