Light-weight system monitor tool/app for OSX, Conky alternatives [duplicate]

MenuMeters

• Official website, free & open-source
• A fork of MenuMeters is being maintained for El Capitan+ including Mojave and Catalina: Maintained Fork Repo

Features:

• CPU, memory, disk, and network monitoring tools for Mac OS X;
• are a true SystemUIServer plugins (also known as Menu Extras);
• Net Meter can display network throughput as arrows, bytes per second, and/or as a graph.
• minimalism: extremely lightweight and dumb simple;
• size is ~1Mb;
• open-source, freeware.

Official website states about exception:

MenuMeters cannot be used on 10.11 El Capitan

But I managed to run on OS X El Capitan (10.11.3) without any problems, I will provide the link on re-compiled version, as this tool is opensource. It works like a charm for a last year, not a single issue found even after automatic system updates.

Configuration screenshot:

iStat Menus

Official Website (single license: $18, "family pack" (5 licenses): $25, FREE trial: 14 days)

Features include (but not limited to):

• CPU usage
• Process usage breakdown
• GPU usage (includes FPS)
• Current uptime
• Memory usage/pressure
  • app/wired/compressed memory
  • swap memory monitoring
• Disk IO (external disks supported)
• Network monitoring
  • Individual interface upload/download, total data transferred, peak speeds etc
• Sensors (temperatures, ambient light, power draw current/volts)
• Battery information (health, capacity)
• World Clock
• (my personal favorite) graphs of the item for the past hour, twenty-four hours and seven days

Size: 58.7 MB
Customization Options:

• the entire layout of the menubar dropdown is completely customizable and displayed in a user-defined layout
• the type of graph displayed can be customised (opposed, centered, stacked)
• the skin of the app can be customised (dark, light, other colors as well)
• menubar footprint is customisable as well

Screenshot(s):

Main overview, CPU, memory, sensors

Menubar footprint

Main app window

Resource usage: iStat Menus is on the low-medium end of the resource consumption spectrum. The app uses about 100-150 MB of RAM at any one time, and has minimal impact on the CPU.

BitBar

Official site (free, open source)

The BitBar app lets you put the output from any script or program right in your Mac OS X menu bar. And it's completely free. An impressive number of plugins have already been contributed by a wide range of developers just like you, and this site makes it easy to find them.

The website does indeed list a large number of network and system plugins for monitoring CPU usage, uptime, battery status,etc., and of course you can write your own; they're just shell scripts.