SLRN used to be be a pretty popular newsreader implemented with S-Lang, never mind that S-Lang, sans the language part, was a quite common alternative to ncurses as a terminal interface.

The language itself is... interesting. C-like, but with some array-based constructs and quite a large amount of backwards compatibility -- there's a "_for" statement and default arguments are weird, as they're not mentioned in the prototype, but just done in the main body. It's a bit like Perl, but without the logic.