One of the most valuable applications in the history of personal computing. I just hope they continue developing it; it's been stuck at v2.0.0 beta 10 for a long time. But at least it's running stable.
I used to use this to display the price of a single stock in my menu bar and refresh every 20 minutes or so - but now that Google and Yahoo don't allow that any more, none of the plugins have been updated to a new source (or a way to use the old ones that doesn't error out) I had to stop using this.
Simple yet feature rich, stable and highly customisable.
The user community is active and lots of plugins/demo-scripts are available for download.
Dark menubar is supported too (use different colours/icons based on your macOS theme)!