21. The Blackjack
After the card game in which players try to get cards adding up to the highest number without going over 21.
22. The Catch
From Joseph Heller's famous 1961 satirical novel Catch-22.
23. The Michael Jordan
The number he wore on his way to becoming the best basketball player the world has ever seen, and one of the most recognized people of all time.
24. The Jack Bauer
The lead character in the television show 24, portrayed by Kiefer Sutherland.
25. The Shave and a Haircut
From the famous lyric, "Shave and a haircurt, two bits." A bit was one eighth of a Spanish dollar, which was in wide use in America in colonial days. When the nation established its own monetary system, it kept the dollar rather than the British pound, but made it a decimal monetary system. Today, it is no longer possible to have one bit (twelve and a half cents), but you can have two—and "two bits" is still used as a slang term for twenty five cents.
26. The Zed
Z is known as "zed" in British English. (It is "zee" in American English, but that's not as fun to say.) It is the 26th and final letter of the alphabet.
27. The Rock Star
The term "27 Club" refers to the phenomenon whereby an unusually large number of blues and rock stars have died at the age of 27. They include: blues legend Robert Johnson, Rolling Stones guitrist Brian Jones, Canned Heat frontman Alan Wilson, Jimi Hendrix, Janis Joplin, The Doors' lead singer Jim Morrison, Grateful Dead keyboardist Ron McKernan, The Stooges' bassist Dave Alexander, Nirvana frontman Kurt Cobain, Amy Winehouse and many more.
28. The Rage
A reference to the rage virus at work in the 2002 zombie movie 28 Days Later and its 2007 sequel 28 Weeks Later.
29. The Leap Year
In leap years, February has 29 days instead of its usual 28.
30. The Carrousel
This is a reference to the classic 1976 sci-fi film Logan's Run. In it, every resident must undergo the ritual of "Carrousel" at the age of 30, where they are vaporized with the promise of being "Renewed." (In fact, they are simply being executed for the sake of population control.)