NASA began the tradition of playing music to wake up astronauts in 1965. Since you don't have the normal sunrise and sunset in orbit, it was considered important to regulate astronauts' sleep time to coordinate with each other and with Mission Control. The very first musical wake-up call was a parody of "Hello Dolly," with lyrics re-written for the mission, sung by Jack Jones. From that time until 2011, the NASA wake-up call has been a surprise for the astronauts, and often a meaningful selection.
No one really knows why a "Hello Dolly" parody was the first wake-up call, but NASA Chief Historian Bill Barry told PRI that it may have simply been an inside joke. Eventually, Houston expanded its musical tastes, piping in an array of tunes during the Gemini 7 mission for wake-up calls and times of inactivity. Because the flight took place in December, many songs were fit for the holidays, including "I Saw Mommie Kissing Santa Claus," requested by astronaut Jim Lovell's 12-year-old daughter.
The Apollo 10 crew woke up to Frank Sinatra's "Come Fly With Me," and heard "Fly Me to the Moon" when the actual moon shined brightly through the capsule's window. For Apollo 11, the mission that carried Neil Armstrong and company to the moon, NASA played news and sports reports instead, but soon went back to music for the Apollo 12 mission. Two years later, Apollo 15 astronauts woke up to the theme from 2001: A Space Odyssey.
Greetings were often taped by celebrities, and some even sing new versions of their songs for the wake-up calls. You can read about the tradition, and see videos of quite a few of the more memorable wake-up calls at Popular Mechanics. -via Digg
One fun story is when one of the astronauts was a graduate of CalTech. There, the 'dead week' tradition is to wake up to the "Ride of the Valkyries" played at full blast by all of the students (the week before finals, when everyone must cram). Someone at the Jet Propulsion Lab provided this music to greet the astronauts on that mission, causing the CalTech alum's vital signs to hit the roof, as he had a flashback to his college days.