nvi2 [0]: I got to like the simplicity of nvi when installing Void Linux on my laptop, but it had some annoying bugs that made me switch to nvi2. In general, it feels like `good' software; powerful enough by virtue of being a 1:1 vi clone with a few crucial improvements (multibyte, multi-undo, etc.), but simple enough to hack on if I miss some feature. Though no autocomplete means it's not suitable for more verbose languages, like Java.
QuickJS [1]: qjscalc is my go-to scientific calculator, and qjs my go-to JavaScript implementation for simple programs. The C interface is very nice to use, too. All in all, it feels very much like a "complete" engine, even if not quite as fast as one with JIT.
w3m [2]: Somewhat lacking as a web browser, but a very good pager. Would take it over less any day. Also has the best table display of any text-mode browser, supports inline images, and is rather extensible.
Wine [3]: It's gotten so good that I no longer have to dual boot Windows. Still not perfect, but definitely on my list of "good software".
[0]: https://github.com/lichray/nvi2
[1]: https://bellard.org/quickjs/