I am curious to know what mechanics exist that can be bolted on to any system.

For example, the Bennies mechanic from Savage Worlds. Or d100 sanity effects...

Reddit/r/d100 is a great resource & community, but it's content is mostly for fantasy games.

Would love to hear about mechanics from systems, or mechanics you've invented.

I GM story & narrative driven games. Combat is often only 25% or so.

Mechanics that relate to plot lines, character development, or social influence would be of most interest to me.

Thanks in advance, comrades.

I didn't try it personally, but I think that you should check 13th age Icons.

Definition and examples: http://www.13thagesrd.com/icons

How to use them: http://www.13thagesrd.com/runn...g_Icon_Relationships

A tip from somewhere else, instead of rolling at the start of the session, you do it at the end of a session and plan you accordingly for the next.

I do plenty, even putting together a couple complete new systems of my own. Any particular mechanics you'd like to hear of first.

One of the best instruments I found is the Emotion Matrix from Tenra Bansho Zero: http://tenra-rpg.com/support.shtml

You roll on the table to give players the first impression for important NPCs (I found out it works for locations as well) when they first meet them.
It's just the impression, so the emotion may change as soon as they first interact, but it's been impressively entertaining until now.
GM and players can change the result bartering points that are used in the game for advancement and roll boosting, but it can easily be adapted.

Obviously, PC roll on that for each other's first impressions at the start of the game.

Aspects from Fate Core work in any game, particularly because they arose from the tendency people have to appeal to the basic descriptions of their characters when the mechanics fail them. All Fate does is put a mechanical limit on how often one can do that, with which parts of a description and to what effect, as well as letting people do it more often the more they contribute to the drama of the game.