What variant rules (not house rules) have you used in your game?

[Alzrius]

So I was recently flipping through some old d20 materials, thinking about all of the interesting tidbits that they had which I haven't (yet) had a chance to actually use in play. That got me thinking about how there's currently a large number of products out there that are add-ons for someone else's RPG system. From supplements DM's Guild supplements to third-party products published under the OGL to stuff on fan-sites, there's a lot out there that takes the "base game" and modifies or adds to it somehow.

Using "variant rules" as a catch-all for material that a third party has written for a given RPG (with "house rules" being stuff that you or your group have invented yourselves, rather than using something someone else made), what variant rules have you used?

 

[Umbran]

My Ashen Stars game uses what some call "popcorn initiative".

None of the games I've seen use it actually call it by that lousy name, but there it is.

 

[aramis erak]

I can honestly say that, for d20, not one rule I've used counts as variant in your definition, as the non-D&D ones have all been whole standalone cores: D20 Star Wars (not Saga Ed), T20 traveller's handbook, and an abortive attempt at d20 Fading Suns, and another at Judge Dredd. Each one, I ran only with its core rules, even if the core could have been done as a variant, because they were a core ruleset.

Now, for Traveller.... Desert Environment, Undersea Environment (both from Gamelords), IISS Ship Files (Games Workshop UK), ship designs from a variety... sectors from JUdges Guild (including their variant encounter tables)