MenuMeters

Formerly developed by RagingMenace, MenuMeters is a preference pane that manages a set of unobtrusive CPU, memory, disk, and network monitoring tools that live in the macOS menu bar.

The most recent version is this fork, and the source is at GitHub.

MenuMeters and other tools that animate menu bar items extensively are prone to making the WindowServer process hog the CPU, which can be fixed simply by turning them off and on again and mitigated by slowing down their refresh rate. This does not appear to be a MenuMeters bug, but rather one of the many unfixed issues in post-10.10 macOS.

A couple of other nice, flexible Open Source menu bar tools are stats (which has a more modern look, and is also Open Source) and AnyBar and bitbar, which allow you to set up visual indicators for UNIX processes and tools.