For folks who speak JavaScript, Feross built WebTorrent which brought the protocol into a browser tab.
The code base is delightful to read. A lot of developers are better at writing code than they are reading it. If you're wanting to flex your code-reading muscles, I haven't found many better places to start than the WebTorrent codebase. I put it up there with redis in being fun to read.
And for Go, there's https://github.com/chihaya/chihaya & https://github.com/cenkalti/rain.