[drmohundro]

Reveal.js (https://github.com/hakimel/reveal.js) also supports writing your slides in Markdown.

I've used it for the last 3 or 4 presentations I've given and it has worked out well. It doesn't come with a huge variety of themes, but it is easy to share - I can just push it out to Github Pages.

[LaSombra]

Reveal.js is practically the default presentation tool for some groups at my workplace. Works quite well and looks quite good as well. Publishing is easy and works on FF and Chrome.