Earlier today, Apple announced they were starting a beta service of iWork applications online, free for anyone (with a licensed copy of iWork '09) to try, called iWork.com. The premise is this: You make a document in, say, Pages, and then click a button to put it online. You let your editor/friend know the document is online, and then he or she can login to the iWork website, view the document as you created it, and put sticky notes on it. This way, the editor/friend doesn't have to have a certain application (like Word) on his or her computer to view or make notes on your document.
Apple's not the first to have a product like this—Google has had its 'Docs' suite of online writing/sharing applications for a couple years already (it's free and it also lets everyone work on the document together)—but they have the opportunity to win the Online Office Suite battle, which has barely begun.