Flipping Heck  

Select a bunch of objects and press R to flip. You can rebind the key. It also appears as an option in the context menu.

Do note that it fails at:
- Preserving wires and other connections if they're not part of the initial selection
- Preserving the undo registry entries
- Being very fast

Why this is not in the base game
It is very slow for a simple "flip" action. The game was never designed with flipping-after-spawning in mind. The way this addon works is that it serialises the selection, and deserialises it flipped around. Almost identical to saving something and then spawning it flipped. It's slow and doesn't even preserve the selection fully. The only true way to make flipping-after-spawning work correctly is to rewrite the foundations of the game and possibly rewrite hundreds of scripts specific to items.

So instead of doing that and likely breaking 99% of all contraptions you have saved, I made this mod. Making mods comes with the privileges of it being OK to release barely working code. It's just a mod, after all.