Custom units and user defined techs?

[Bestbank Tiger]

I was going through the Civ files and I noticed they left spots for you to add your own techs to the tech tree and create your own military units,

What has everyone done with this? I like tweaking the game as long as it doesn't radically affect gameplay.

Thanks.

 

[ElephantU]

An amusing one I heard was to set Masonry and Philosophy as pre-reqs for Plumbing, and then make that a pre-req for Labor Unions.