I'll pull up a full post of what I believe to be all of the "free" mega events in a bit.
List of Current Mega Events (what I believe to be DLC will be noted):
Anomalies (Anomalies are spawned on the map)
Artifact (Weakest faction gets a per-turn power up till the end of the game) (DLC)
Assassination (A leader of a faction is assassinated, plunging the galaxy into war) (DLC)
Breakthrough (Random tech given to all factions in game)
Distant Resources (More Antimatter and Elierium is discovered)
Dreadlords (The Dreadlords spawn on a planet) (DLC)
Gold Rush (Anomalies are spawned on the map [not as many as the first one])
Land Rush (Several Class 0 planets turn into Colonizable ones)
Paradise Worlds (Several Class 26 planets are discovered)
Peacekeepers (The peacekeepers have arrived and will make sure there is peace in our time) (DLC)
Pirate Colony (Pirates tale over a Class 0 planet; hijinks ensue)
Relics (Several new relics spawn on the map)
Resource Glut (More Duarantium, Promethion, and Thulium are discovered on the map)
Space Monsters (Space Monsters appear on the map) (DLC)
Wormholes (More wormholes are spawned)
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I think that's everything. If anyone can point out an error, please let me know. 