I like wargames and boardgames and boardwargames and over the years I've come up with various variant or expanded rules.

Space Hulk

Space Hulk was an ace aliens-in-corridors board-game by Games Workshop.
Space Hulk Complete Weapons List
Space Hulk Variant Psychic Powers

Advanced Heroquest

Advanced Heroquest was an okay-ish fantasy dungeon board-game by, uh, Games Workshop.
Expanded Heroquest Dungeon Generation
Expanded Heroquest Furnishings
Expanded Heroquest Hazards
Expanded Heroquest Traps
Expanded Heroquest Treasure
Expanded Heroquest Monsters
->Expanded Heroquest Dark Elves
Expanded Heroquest Hero Abilities
Expanded Heroquest Chaos
Expanded Heroquest Equipment
Expanded Heroquest Quests
Expanded Heroquest Magic

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Zeroquest; rules for playing heroes and dungeons in a futuristic setting.

Dungeons and Dragons

I've played D&D since it's first edition, but never really thought it was much good as a system until the current 3rd edition.
Dungeons and Dragons monsters