Curl maintainer Daniel Stenberg:

We can’t know for sure which products are on the top list of the most widely deployed software components. There’s no method for us to count or estimate these numbers with a decent degree of certainty. We can only guess and make rough estimates – and it also depends on exactly what we count. And quite probably also depending on who‘s doing the counting.

He goes through his process of trying to determine contenders, and ends up with zlib, qlite, and libcurl. Other have added OpenSSL, expat and the Linux kernel to that list.