From what I've read about Tonic, they have a lot of tech for Powerpoint compatibility--in particular, reading Powerpoint files, writing powerpoint files, and converting them into file formats which do not require powerpoint.
Slide.com doesn't do anything with PowerPoint. If you go yo Slide.com, you can see that they are not a presentation software company; they are a social networking widget company. Slide.com is not about technology--in fact, their tech looks downright trivial. They are all about branding. Slide.com has no technology to buy; they only (maybe will sometime) have customers to buy.
Zenter is an web based presentation app that promises to really take advantage of being online. Users will have the regular functionality of PowerPoint, but with the ability to directly add content from the web (Google Images). Each public slide show will also be put into a public library, for other users to remix or just drop into their show.
