The three things that I am aware of are:
The
Fabled named mobs (upgraded versions of the original named mobs) that appear in various old world zones. They drop Fabled gear, which are upgraded versions of gear that their non-fabled versions drop (or used to drop, in the case of some mobs like Drelzna/Fabled Drelzna).
Then you have the
5th anniversary scavenger hunt which results in a cool fireworks clicky staff that is fun at parties.
And you have the skeleton armies, which are mostly a nuisance (depending on your level) except for
this guy who drops some nice no-drop gear for various slots. These guys don't stay in the realm of the unliving for long.
There may also be random GM events, but I've only seen one (a giant-sized dwarf with a pet basilisk named Fluffy in butcherblock) and as far as I know he only handed out food and booze. It was neat seeing this huge dwarf stomping around tho.
There might be other stuff, but I've not heard anything about it.