The team at Kickstarter made a simple, synchronous event sourcing library implemented in Ruby.

We’ll go over a high level introduction to Event Sourcing where we will highlight the four components that make a (minimal) Event Sourcing system: Events, Calculators, Aggregates and Reactors. We will then talk about how we implemented a (minimal) Event Sourcing Framework at Kickstarter for d.rip. And finally we’ll reflect a bit on the ah-ha moments and the challenges that we’re going through with this approach — 9 months after having started to work on d.rip and 4 months after launch.

https://kickstarter.engineering/event-sourcing-made-simple-4a2625113224