SQLite is likely used more than all other database engines combined. Billions and billions of copies of SQLite exist in the wild. SQLite is found in:
SQLite is perhaps one of the top five most deployed software modules of any description. It might even be the number one. Other libraries with similar reach include:
(If you know of any other software that should be added to this list, please send a quick email note to firstname.lastname@example.org. Thanks.)
Libc is omitted from the above list even though it is more common than SQLite because libc is not a single software component but rather several competing implementations (ex: BSD vs. GNU) with similar interfaces.
Precise numbers are difficult to obtain and so exact rankings are impossible. But some commentators observe that SQLite tends to be statically linked and thus have multiple instances on each machine, whereas libz, libpng, and libjpeg tend to have just a single instance per machine in the form of a shared library or DLL. So even though the number of devices containing libz, libpng, and/or libjpeg, may be greater than the number of devices that contain SQLite, the total number of instances per device might be higher for SQLite and so SQLite might be the single most widely deployed and used software component.