Oberon. :-)

It's the granddaddy of all of them.

Overview: http://ignorethecode.net/blog/2009/04/22/oberon/

Academic paper (but highly accessible): https://pdfs.semanticscholar.org/d48b/ecdaf5c3d962e2778f804e...

Yes, it's an OS as well as a language although a tiny one that runs under MacOS, Linux and Windows if you want. So does the bare language of the same name. (Think of it as Pascal 3.)

The inspiration behind Plan 9's Acme.