Matt Lloyd offers a set of Basic Skirmish rules with dungeon crawl adventures in mind.
Rules: Fantasy
100 Kingdoms Fantasy Rules
100 Kingdoms is a set of free wargames rules for large scale fantasy battles.
De Bellus Middle Earth Rules
De Bellus Middle Earth is DBA variant for playing wargames set in JRR Tolkien’s Middle Earth.
Dragon Fight Aerial Combat Rules
Dragon Fight is a set of free wargames rules for aerial dragon combat.
Bladestorm Quick Start Rules
Bladestorm, the classic fantasy wargames rules from Iron Crown Enterprises in the 1990s is back in a second edition. You can get the quickstart rules at the link.