Thumbstacks would be a good start, if you're going to build an Office-killing web-based suite with a PowerPoint killer. But, Thumbstacks is a serious lightweight compared to its older cousin, but still slick and easy to use. Some of the functions are a bit hard to find (themes and images, for example, are hidden in the right-click menu), but people used to PowerPoint's WYSIWYG interface will find themselves more or less at home. Presentations created with Thumbstacks are hosted on Thumbstacks' servers, which makes accessing them from anywhere easy, but also raises a few concerns about lock-in (what, no PDF export?). On the upside, it does support printing in a variety of layouts and even has an "Add photo from Flickr" option.