Regular Expressions: I think this is what made Perl and the Unix tools grep, sed, awk, etc. super popular and useful.
Web server / client: This is what has made Go so extremely popular as the Cloud-native and Microservices language of choice. Their HTTP implementation is ingeniously designed to make it truly upgradable when new versions of HTTP come out. It's impressive how they could go from HTTP/1.1 to 2 and 3 without practically touching the public API. With this in place, come the next level killer apps in the form of web frameworks.
Code analysis / suggestions: Rust's Clippy is impressive in its code analysis capabilities, when to collapse nested ifs, when you're comparing floats for equality, etc.
Fuzzing for tests.
Benchmark suites with cool graphs.
Profilers with cool graphs.
Improved editor plugins, syntax highlighting, snippets, templates, etc.
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ifs, when you're comparing floats for equality, etc.