I chose Zoho Show for it's ability to host PowerPoint presentations. I use ppt a lot for my classes, and while Internet Explorer tries to allows users to view the slideshows, it often causes more grief than anything.
Zoho seems to work easily enough. I especially like the functionality of exporting files to other formats (.pdf for starters). I could definitely see myself using this as an addition to my classroom website for students who might be absent during a lecture. I'll have to play around with it some more, but I would also like to be able to add audio recording as part of the presentations.
The other functions seemed useful, but I already incorporate much of what they do with Microsoft Outlook and my smartphone.