VoltDB took this to an extreme - the way you interact with it is by sending it some code which does a mix of queries and logic, and it automatically retries the code as many times as necessary if there's a conflict. Because it all happens inside the DBMS, it's transparent and fast. I thought that was really clever.

I'm using the past tense here, but VoltDB is still going. I don't think it's as well-known as it deserves to be.