NS-3 network simulator uses 128bit integers to measure time and they have 3 implementations (search page for int64x64): http://code.nsnam.org/ns-3-dev/file/38d46996c708/src/core/mo...

It would be much easier for them to just build on top of i128.

Also, you can see that one of the implementations is copied from Cairo, so it seems graphics libraries have some use for 128-bit integers too.