Easily create Reveal.js slides online
Reveal.js is a free and open source framework for creating presentations using HTML, which can easily win over PowerPoint. The only problem associated with it is that you have will be needing basic knowledge of HTMl to start with it.
Slid.es is an online website where you can create presentations on Reveal.js for free, without having any knowledge about HTML. You can create slides, add animations, select theme, put pictures, change background color or picture, add formatted text etc.
Only Pro users can download the presentations for offline usage though.
Many of us upload PowerPoint presentations to Slideshare.net, as this service makes the slides available on almost all platforms, but with a compromise on slide transition effects and a presence of a bottom control bar even on full screen mode.
If you are going online, Prezi is also a good presentation software, which makes use of Adobe Flash. This makes it not compatible on devices which does not support/have Flash.