What are some alternatives?
When comparing Beslide Me and Marvel for Keynote, you can also consider the following products
Sidekick - Email tracking plugin for Gmail.
Slides - Slides is a place for creating, presenting and sharing presentations. The Slides editor is available right in your browser. Unlike traditional presentation software, like PowerPoint, there's no need to download anything.
SlideLizard - Powerpoint plug-in for live audience interaction
SlidesPPT - Beautiful free presentation templates for Google Slides