Have you looked at https://github.com/Microsoft/monaco-editor
They have examples of what it can do at
https://microsoft.github.io/monaco-editor/
It literally took me a couple of days to integrate their diff viewer into my product. If you go to
http://bitbucket-server-demo.gitsense.com/plugins/servlet/gi...
and click on the last tab (077be8bd -> 966c0823) and click on any of the files in the tab, you can see the diff editor in action. I don't know how much money was spent in developing the editor, but it's FREE for use by the community.
Z3 (https://github.com/Z3Prover/z3) is the Satisfiability Modulo Theories (SMT) solver to use. It is such a quality piece of software and it's amazingly easy to find help on StackOverflow from the main developers. Z3 really is the default choice for an SMT solver these days (i.e. you only reach for other solvers when you really have to).
Roslyn (https://github.com/dotnet/roslyn) is the C# (and VB) compiler as a library. Useful for doing code analysis and code generation for a modern imperative language. Roslyn is a whole lot easier to get started with than integrating Clang/LLVM into a project, so it really lowers the barrier of entry for lightweight program analysis projects.
'very' useful is of course subject to debate, but did you even bother to check what they have on github? F#/msbuild/vscode to name some bigger ones, then there's smaller things like ffmpeginterop etc which I was looking into just last week.
Uhm... .NET? Anyone?
Google doesn't open source their search engine, most larger corporations don't open source some key products that they make money from. They are in the business of making money, afterall.
Edit: to be clear, I know it is in their best interest not to open source those products, because that's where most of their money comes from. But it really looks to me like the want to flood the market with random open source stuff, so that they seem more open source friendly. But in reality they are not actually contributing anything very useful to the community.