Behind the scenes: Before
the Moog analog synth allowed patching together music with knobs and
telephone switchboard cables, Delia Derbyshire (shown) and assistant Dick Mills
started with a simple score by Ron Grainer and pieced together one of
the most recognizable TV tunes of all time using individual sound
modules, razor blades, an enormous amount of work, and a lot of
imagination. (Read the detailed history by Mark Ayres, who composed for the show and produced a CD re-release; here are some of my favorite highlights — read more)

• 

  1963 original equipment: 12 test-tone generators (not
  originally intended for musical use!), a couple of EQs, an LFO, and a
  noise generator; pieced together by hand without keyboards onto tape
  (no, really) — the result is incredibly organic, and probably impossible to reproduce with today's tech!

• "Cliffhanger" screech: Again, according to Mark Ayres' superb write-up, the screech was produced by gradual tape frequency shifts with delay
• 1980 Peter Howell version: This
  brilliant remake employed a hit list of great gear: Yamaha CS80, ARP
  Odyssey, EMS Vocoder, Roland Jupiter 4, and additional tape — plus
  some samples of previous Doctor Who effects and sound. I think this is
  probably the most underrated of the versions of the song; while not as
  significant as the original, it translates its spirit to a later
  generation of synthesis
• Dalek speech: Normally, if you want to tell an English
  audio student what a ring modulator is, you point to the Daleks, though
  Music Thing has an article
  on the debate over that: basically, ring mods are used now (and I would
  guess on many later shows) but would have been unlikely in the 60s
  programs (see discussion of the Moog sighting on the new show).
• More Delia Derbyshire: Mark Ayres is features in a BBC 4 program (info and video clips)
  featuring Derbyshire's catchy ditty "Ziwzhi Ziwzhi oo-oo-oo" — a tape
  reversal trick that produces a bizarre song by robots worshipping a
  power converter

And horrible Doctor Who variant songs:  Go buy the CDs and give the BBC its due (and hear the music well-produced), but Clive Banks' extensive TV site has some oddities you probably won't hear elsewhere: (aside from the 1963 original and 1980 remake)

• Even Delia Derbyshire didn't like the "Delaware" theme (Pertwee era) — proof positive that it's not easy to make a good Who theme
• Try to act surprised when I tell you the composer of this overblown theme for the US movie flop also did Passion of the Christ
• And you thought only William Shatner did bizarre spoken word "music"? So did Doctor #3 Jon Pertwee
• Apparently the Doctor is in distress (as will you be when you hear this)
• K9, the song — need I say more?

Oh, and let's see how long it takes BBC's lawyers to notice the new series theme is up there, too.