Drew Devault made a website to track simplicity of projects, (supposedly) based on the functionality rather than softwares names: https://arewesimpleyet.org.

It sorts software by functionality (compilers, shells, libc, etc..) and references "sane" implementations.
I like the concept, unfortunately this is from his point of view, and the list is very (very very!) opinionated (no mention of suckless tools like scc, sbase, ...), and seems to serve as an advertisement for his own software rather than an exhaustive list.
This is a good starting point though, if you seek for simple software.

Now we can discuss what "Simple" means for software: simple to use? or simple to code/review?
I'd strive for the 2nd, as the latter is more easy to judge. Usability being such a subjective topic.