I've been discovering more and more high quality open source apps for Mac recently. My other favorites so far:

https://github.com/iterate-ch/cyberduck

https://github.com/rxhanson/Rectangle

https://github.com/sfsam/Itsycal

https://github.com/newmarcel/KeepingYouAwake

I would like to add stats[0] to the list, which is a great open source replacement for iStat Menus

[0] - https://github.com/exelban/stats