Initially his blog, Joel on Software. Then his books. A big part of Stackoverflow's success was programmers already knew who Joel and Jeff were.

He and his team created Stackoverflow, a site used by every developer every day. He's a prolific writer around software engineering & managing software engineering. He's got an incredibly blunt writing style w/o any fluff and his writing on topics such as interviewing, refactors, and technical debt are shared around often.

Two decades of experience building great software. Including Microsoft Excel, Trello and his obviously most important contribution: Stackoverflow.

He cane up with the Joel Test (https://www.joelonsoftware.com/2000/08/09/the-joel-test-12-s...). It's still incredibly relevant. A few years ago job vacancies would include things like 'we score 11 on the Joel test, come work for us'. I haven't seen it for a while, but I still do it mentally when I go to a new gig

He was very skilful at telling developers things they wanted to hear in his blog. But until Stack Overflow I didn’t know a single person who used any of his products (FogBugz, CityDesk) and it didn’t seem plausible that the hottest developers in the world would be beating down his door to work on them.

The current stupidity of the hiring process, the inane whiteboarding, the take-home tests and all that, is all down to him.

He co-invented the website which all developers use on a day to day basis?