I have some oldies in mind from the time when the terms expansion/DLC didn't exist yet, they were called Add-ons:
- Commandos: Beyond the Call of Duty - several new great missions
- Carmageddon: Splat Pack - the new tracks weren't that great IMO but still it was more of Carmageddon
- Heroes of Might & Magic II - The Price of Loyalty? - just more maps to play
Deus Ex (2000) - a game that pushes the boundaries of what the video game medium is capable of to a degree unmatched to this very day.