I'm pretty sure the (here much maligned) Eve programming language was an example of this. It was posted a few times before its demise. I'm also a fan of the paradigm, particularly Eve's relational/behavioral style.
Not quite behavioral (which is a very specific thing), but Eve was certainly synchronous. It was one of the two languages (the other being Céu) that brought synchronous programming from the niche of safety-critical realtime systems to more mainstream software. IMO, it was the most theoretically-advanced programming language around.
