Oh I have quite a few.
#1 a BASH script automating ~70% of all our team's work back in my sysadmin days. It was like a Swiss army knife. You could even do `ScriptName INC_number fix` to fix a handful of types of issues automagically! Or `ScriptName server_name healthcheck` to run HW and SW healthchecks. Or things like `ScriptName server_name hw fix` to run HW diags, discover faulty parts, schedule a maintenance timeframe, raise a change request to the appropriate DC and inform service owners by automatically chasing them for CHNG approvals. Not to mention you could `ScriptName -l "serv1 serv2 serv3 ..." doSomething` and similar shit. I am VERY proud of this util. Employee liked it as well and got me awarded. Bought a nice set of Swarowski earrings for my wife with that award :)

#2 a JAVA sort-of-lib - a ModelMapper - able to map two data structures with a single util method call. Defining datamodels like https://github.com/netikras/... (note the @ModelTransform anno) and mapping them to my DTOs like https://github.com/netikras/... .

#3 a @RestTemplate annptation processor / code generator. Basically this dummy class https://github.com/netikras/... will be a template for a REST endpoint. My anno processor will read that class at compile-time and build: a producer (a Controller with all the mappings, correct data types, etc.) and a consumer (a class with the same methods as the template, except when called these methods will actually make the required data transformations and make a REST call to the producer and return the API response object to the caller) as a .jar library. Sort of a custom swagger, just a lil different :)

I had #2 and #3 opensourced but accidentally pushed my nexus password to gitlab. Ever since my utils are a private repo :/