C, C++. Compilation can be multi threaded. Linking is inherently mono threaded. A number of optimizations flags are mono threaded (and take very long).

Python. Lint is mono threaded. The virtualenv setup is mono threaded (running requirements.txt).

Java. Mono threaded for package generation (jar files). Not sure about compilation and running tests. Most of Eclipse is mono threaded (last I used it) like indexing and rendering and auto completion.

Outside of that, opening any IDE, tool, application or browser is a mostly single threaded operation. The rendering of most GUI is also single threaded.