S5 has gotten less attention than it should have, once I realized what S5 stood for. To start, you can say S5 is powerpoint done through the web. As long as you have an internet connection, you can make, edit, and share powerpoint presentations very easily.
I didn't think much of it at first, since a lot of people are simply taking desktop applications and putting them on the web. Writely is a word processor on the web. There's someone else doing a spreadsheet that I can't remember the name of.
And presentations would transfer easily from place to place...given that the network was available. I think it'd be useful when wireless internet connection is ubiquitous.