We recently dug out our old Epic figures for a game and had a lot of fun. Lots of figures are still available on Ebay and you can get the Epic Armaggedon rules at the link: Epic Armageddon Rules and Army Lists
I stopped buying Warhammer 40K stuff when they dropped the squats as a major race. In my basement workshop, I’ve got a horde of the stunties, in regular battle gear, on trikes, in exo-armor … you name it. You can find an online squat codex in the War Vault.
Games Workshop offers a free pdf of its Warhammer 40K battle primer, which contains basic for moving, shooting, fighting, using psychic powers and morale, along with other material to get you started.
Warhammer Star Trek is a fan-created modification of the Warhammer 40K rules. I ran across these when looking for Star Trek rules to use with my newly painted 15mm figures.
One Page 40K Rules are a set of simple rules for the 40K Universe. The authors write:
One Page 40k is a single-page ruleset designed to be fast to learn and simple to play. It serves as a great introduction to the Warhammer 40.000 universe for people who are new to the game, and as a way to play quick games for veterans of the franchise.