Mac OS X Lion has plenty of features that make it worth the upgrade for all Intel Mac users, but it's still missing one of my favorite extensions from the Mac OS legacy: Oscar the Grouch.
Oscar was a trash-bin animation utility that originally came bundled with OS 7.1 (later updated in 7.5) and featured everyone's favorite grumpy sanitation officer singing "Oh I love trash" and "I love it because it's trash" whenever you emptied the bin.
The character cameo quietly disappeared soon after, although Henson and Kermit the Frog himself would later appear in an Apple commercial for its first iMac desktops in 1997, not to mention the company's famous Think Different ad campaign.
Incidentally, if you're in the New York area, be sure to check out the Jim Henson exhibit currently running at the Museum of the Moving Image in Astoria. Entitled "Jim Henson's Fantastic World," the retrospective includes more than 120 artifacts that span the artist's career--including "Sesame Street," "The Muppet Show," and "Fraggle Rock."