Hi Derek,
I like this one: https://github.com/hakimel/reveal.js
It's browser based (Javascript, and therefore web friendly and platform agnostic) and it has some nice features, like slides with sub-slides (2D slide layout), and a zoom-out mode so you can navigate directly to slides via a "thumbnail map."
It takes a while to learn all the keyboard shortcuts to make it really useful for live presentations though. You need to know some basic HTML to create new slide shows on it, so not particularly user-friendly.
It may not be everyone's cup of tea.
I hope this helps!