I use the FMC - Fundamental Modeling Concepts language to draw systems and architecture diagrams. The beauty of FMC is that only two node types exist, both having very clear semantics (depending on which of the 3 diagram types you draw/read). In a systems diagram a sharp rectangle represents an actor and a rounded rectangle/circle represents a persistent/volatile memory. You can only ever connect an actor with a memory (space), I think it’s called a bipartite graph.
My university took part in its research (less nowadays) and I’m really glad I had the chance to learn it. I still use it at work.
Also a big fan of FMC for architecture diagrams - in bigger systems, being able to easily distinguish "action" vs "state" nodes has been useful in reasoning about behaviors. Pity it never took off, and there doesn't seem to be much tooling for it.
For other uses - yeah, ERD and sequence diagrams are the workhorses.
My university took part in its research (less nowadays) and I’m really glad I had the chance to learn it. I still use it at work.
[0] http://www.fmc-modeling.org/