We don't know any good Spectacle open-source alternatives yet.
You are more than welcome to suggest any alternatives that you know of, or ask the community for suggestions.
Alternatively, we recommend checking out LibHunt for open-source libraries and alternatives to Spectacle.
I started using Spectacle during their beta. It has saved me both time and money in creating support video for my SaaS product.
Really novel approach to a super annoying problem. We love it!