HTC Sense is a UI (user interface) built by HTC and put onto many of there android and windows based phones and debuted on the Desire and Legend. HTC brag that Sense is a "design experience" thats it "is all about how people use the device, making their content personal to how they use the device." One of the main selling points of the HTC Sense based phones is the interaction between itself and social networking sites like facebook and twitter. HTC Sense has 7 home screens for you to add widgets and shortcuts on, HTC have created there own widgets including Mail (left screenshot) Friendstream for you to see you friends facebook and twitter updates in a time-line and the clock you can see in the right screenshot. HTC Sense is set to revoloutionise how we use our phones and even the relationships we have with them.
Click here to see the very impressive HTC Sense launch video altough this is a slightly older version then the most recent: http://www.youtube.com/watch?v=MchNr8TagNU&feature=channel