If you missed Apple's event where the company has announced the iPad Air, the Retina display iPad Mini and some other product updates, then you can watch the entire keynote on Apple’s website (follow the link from the source). Even if you did follow the event, maybe you want to see it again, maybe to watch Eddy Cue and his poster for his musical event or just to see the crowd's reaction when the iPad Air is announced.
To see the event video online, you will need Safari 4 or later on OS X v10.6 or later; Safari on iOS 4.2 or later; or QuickTime 7 on Windows. Or maybe you want to sream it via your Apple TV, the second or third-generation Apple TV with software 5.0.2 or later will do it. Let us know what was your favorite moment from the keynote.
Source: Apple Special Event page