There is no dearth of mind-blowing open source stuff available for web developers in this world. However, this is mostly limited to "raw material" stuff like CSS Templates (Bootstrap), JS Frameworks (React/Vue/Angular), Images (Pexels) and Icons (Font-Awesome/Google Fonts).

But when it comes to "ready made" stuff like dashboard templates (with seamless integration of charts, calendars, grids, tables, etc.), even there you have a few options. Three such open source projects I know about are:

1. AdminLTE - This is by far my favorite because their open source version is "truly" open source, they don't hold back stuff like calendar and chart plugins to lure you to the pro version. While they do have some premium templates, it's mostly custom styling on top of the base version, not any extra features.
2. Material Dashboard - This is also a good one (in fact, most people prefer this to the above looks wise!) but I don't consider it true open source (as they have plugins for charts, etc. only in the "pro" license). The interesting thing though is that their older version based on Bootstrap-4 is still fully FOSS even with the bells and whistles, wonder why that is!
3. SB Admin 2 - This is yet another popular dashboard template in the open source world. Like above, there are some restrictions in this one too as things like "extended components" and "custom CSS utilities" are pro only! They also won't provide a fixed sidebar in the free version which is almost the bread-butter in a dashboard template.

These three are the ones which are quite popular and I happen to know of. Please comment and let me know if you know of others.