If you've used anything that runs on more than one machine, you've relied on Lamport's work. His 1978 paper on clocks explained that in a distributed system there is no single "now", and showed how to order events anyway.
He also wrote Paxos, a way for unreliable machines to agree on a value. He explained it as a parable about a parliament on a Greek island, and reviewers didn't get the joke, so the paper sat around for years before people noticed it was important. Now variations of it run inside most large cloud systems.
On the side he made LaTeX, which most scientists still use to write papers. He also keeps insisting that programmers should think in math before they write code, which people mostly ignore.