Is there any more widely deployed software in the world than SQLite? Multiple copies (browsers, language runtimes, embedded in other software) on many computers and built into most smartphones and its probably in quite a few TVs and other devices too.
Very successful invisible (to non-developers) software.
Not just successful, it's really good too! The API is rather easy to use, it's lightweight and (for small to medium sized databases) it performs really well. Not really suprising it is so widely used.
A funny thing here... If you count the number of active copies, this may not be true. Usually you don't see ten apps in one system coming with their own version of libc. This may pretty well be true for sqlite.
I suspect it is less deployed than SQLite. SQLite gets into things even without network capabilities (e.g. Digital TVs even before they were internet connected for the channel data and the EPG information).
I don't know to what extent the code of the BSD stack remains in Windows but you can bet even if a raw build of Windows doesn't have SQLite embedded somewhere within it (which it might well do) that there are probably multiple software installations with it embedded somewhere.
Would you actually describe the BSD networking code as being deployed in Linux even? I know it is derived from the BSD stack but is it really still the same software.
Zlib and libjpeg as someone else suggests are good suggestions and at least in the races with SQLite.
Very successful invisible (to non-developers) software.