Microsoft PowerPoint is so commonly used as presentation that its in danger of losing both capital letters. It wasn't always that way, of course (Anyone remember Harvard Graphics? I was actually surprised to find out they were still in business), but by packaging PowerPoint in Microsoft Office, it introduced people to presentation software and quickly took over the market.
Now, there's an alternative. And, not surprisingly, it comes from Google.
Google is using its Google Apps to challenge Microsoft. These are Web-based word processing and spreadsheets. They are nowhere as flexible or powerful as Office (There was a story a few weeks ago that even Google doesn't use them that much), but have some nice features: you can work on files from any Internet connection and you can share your files with others, allowing multiple people to work on a file from multiple locations.
Now I find that sort of thing somewhat useful, but not a killer app. But with Google Presentation, it's a big leap in utility. After all, you often have to do presentations on the road. If it's on Google, all you need is an Internet connection. No need to bring a laptop or flash drive, or worry about compatibility.
In addition, Google Presentation has a chat feature. You could put up a presentation, have people see it and comment. You could also paste a narrative.
It is limited -- fewer fonts, fewer templates, fewer bells and whistles. But it seems set up to take some of the thunder away from powerpoint.