Seconding redis, and noting that the original redis source code is _also_ an interesting read and demonstrates how approachable well-written tcl is, even for those entirely unfamiliar with it as a language: https://gist.github.com/antirez/6ca04dd191bdb82aad9fb241013e...
I also remember how Qt UI code and docs were a revelation after the nightmare of win32 and related frameworks.
Bellard's original Tinycc was very entertaining and took a while to digest.
Most old and well-maintained projects are worth learning from.
Apart from some local tips and tricks what is more important is understanding that there's nothing special about other people's code.