Originally Posted by EebstertheGreat
For future reference, the marker ability belongs only to wards, siege units, and all dummy units, as well as heroes under the effect of certain invulnerability spells.
And couriers (both flying and un-upgraded), all techies mines, psionic traps, tombstone, scarabs (Urna), beetles (the swarm), and fountain.
Most non-dummy units with marker can be found in this list:
Unit Statistics - DotA Mechanics
The big exception being things like Puck in phase shift.