The workflow engine battle has intensified with some more interesting entries lately! Here are a couple I encountered in the last few days. I love that at least two of them are direct references to Luigi!
Pinterest just open sourced Pinball which seems like an interesting Luigi alternative. There's two blog posts: Pinball: Building workflow management (from 2014) and Open-sourcing Pinball (from this week). The author has a comment in the comments thread on Hacker News: