SQLite is used by literally millions of applications with literally billions and billions of deployments. SQLite is the most widely deployed database engine in the world today.
A few of the better-known users of SQLite are shown below in alphabetical order. This is not a complete list. SQLite is in the public domain and so most developers use it in their projects without ever telling us.
Adobe uses SQLite as the application file format for their Photoshop Lightroom product. SQLite is also a standard part of the Adobe Integrated Runtime (AIR). It is reported that Acrobat Reader also uses SQLite.
Apple uses SQLite in many (most?) of the native applications running on Mac OS-X desktops and servers and on iOS devices such as iPhones and iPods. SQLite is also used in iTunes, even on non-Apple hardware.
The increasingly popular Dropbox file archiving and synchronization service is reported to use SQLite as the primary data store on the client side.