ClearType Text in Longhorn
Ian Griffiths (via Chris Sells) is all excited about the improvements to ClearType in Longhorn. It looks like Longhorn is combining the effects of both ClearType and antialiasing. ClearType works by treating each RGB color component as a separate pixel, effectively increasing the horizontal resolution of the screen by a factor of 3; however, it does not produce improvements vertically. Antialiasing, on the other hand, works by alpha blending the edges of text; for small text, this may blur the font somewhat, negating some of the benefits of increased information displayed on the screen. The two approaches complement each other; ClearType offers antialiasing more resolution to work with horizontally; while antialiasing adds vertical smoothing to ClearType (vertical blur is less problematic as fonts tend to be twice long as they are wide).
As a result in advances in text smoothing in Longhorn, text is going to be extremely readable and close to the quality of printed text. Combined this with the improved support for high-resolution displays in Longhorn, and the day of the jaggies will soon be over. Longhorn also sports many textual improvement--for example, more typographic features like ligatures and kerning .
Check out this David Brown's WinHec presentation on Avalon Text for a dramatic illustrations of the differences between Windows XP text and Longhorn text.