Feel free to add inventions and discoveries from any domain and any period of time. We take things for granted, but without them life will be so much harder.

I'll start with the Roman Empire Period, and mention only a few of them:

-aqueducts
-triumphal arcs
-fast curing cements
-reinforced concrete (!!!!!!!!)
-shorthand and symbols such as "&" or abreviations such as "etc.", "NB", "PS" and many others.
-Bikinis (see a famous mosaic from Sicily showing young ladies in bikini)