Messenger for the Web is for when you’re away from your own machine – you know, like when you’re traveling. Or when you want to IM at your friend’s house. It’s also good for professional environments where installing software on company computers is frowned upon, since there’s no download. And Messenger for the Web won’t pop up windows while your boss is standing behind you! It’s got a foolproof boss detector expressly for this purpose. OK maybe not, but in Messenger for the Web, conversations take place in the browser window, which you can always minimize or tab. For privacy and stuff.