[mtnGoat]

stairs also fall into this category.

[bartread]

Also, lots of simple tools and weapons: knives (and blades in general), hammers, bows, hand drills. Candles are almost unbelievably useful when you have no electricity, which leads nicely onto the various methods and tools for firestarting.

[pavel_lishin]

A knife might fall under a discovery, rather than an invention; a piece of flint that fell and chipped just the right way was likely the first knife.