Brick House by The Commodores 
Comfortably Numb - Pink Floyd
The Imperial March.
The Metallica version.
(Yakety Sax would be more appropriate, but still.)
Mine’s “Peaches en Regalia” by Frank Zappa.
I always hear a song like Bela Fleck’s “Flight of the Cosmic Hippo” when I read posts by roger thornhill. So smooth and precise, with a touch of whimsy thrown in, the tune suits him.
When I see posts by Moe I tend to hear some vintage Grateful Dead cosmic jamming that leads into a rendition of Terrepin Station. Although if his post is weird enough, I’ll hear Genesis’ “Supper’s Ready”
I always hear a trumpet fanfare when I see The King of Soup. Then I start imagining an existing tune with it’s lyrics changed so the song is about soup. I recently did this with the song “Man-eater.”
Excitable Boy by Warren Zevon.
hey! where did everybody go?
“All You Need is Love”, Lennon and McCartney.
I had to think about my favourite posters and what I hear in my mind when I see them; spent some time browsing, and came up with this:
Anaamika: I’ve said it before, but I always hear “Dominique” from The Singing Nun. Hee. 
Roland Orzabal: Sure, most people might think of “Shout”, or “Mad World”, or “Pale Shelter”… me, I think of “Bloodletting Go”, because it is my favourite Tears For Fears song, so whenever I see him posting, I think, “Oh yeah, I should queue that song up.” And so it is.
Scott Plaid: Whither art thou, Scott? The one poster who always makes me smile by reminding me of Nik Kershaw’s “The Riddle”. One of the few posters who not only knows what the hell I’m talking about, but seems to like the song as much as I do, too! How rare.
Quicksilver: “For Love”, by Lush.
There are more, but I have to see them posting again to remember. If I try too hard, I forget. 
4’33’’ by John Cage.
You can click on the link for a midi version if you’re unfamiliar with the work.
I’ll take the instrumental Axel F without the crazy frog sound effects, thank you.
For my friend, swampy, I think Teddybears’ Picnic would be appropriate.
Anything rousing, grand, martial, or anthemic - played on a kazoo.
On the humorous side, I’ve always held a love for Adam Sandler’s 7 foot man… (I’m 6’6")
Well I dunno if I have a more serious one, the song which often works its way into my head is Flogging Molly - Devil’s Dance Floor…
Panhandlin’ Prince - Ugly Kid Joe
BigBlock – Jeff Beck
Kashmir - Led Zeppelin
Shucks. How’d you know? It’s one of my favourites for sure. Have it on one of my best spin class workout CD’s.
A recent friend gave me a new theme song. “Your Woman” by White Town. Appropriate on many levels: "So much for my high brow Marxist ways… "
If I had to pick just one for myself… it would be tough… but today, I have to go with “Policy of Truth” by Depeche Mode. No wait! “Like a friend” or “Ladies Man” by Pulp. Unless I’m all like, “Karma Police” by Radiohead.
See?.. I can’t pick just one…
As of late, it’s …
“Alone again” by Dokken.
Chopsticks…played on a toy piano, or a Sousaphone.
The first thing that comes to mind is “Dirty Little Mind” from Jackyl. Then “Subdivisions” from Rush. Followed by “Creep” from Radiohead or “Manic Depression” by Hendrix (depending on my mood, of course).
My husband would say either “Fat Bottom Girls” by Queen or “Girl’s Got Rythm” by AC/DC. Perhaps his theme song would be “Baby Got Back,” Sir Mix Alot. 
Like QuickSilver said, I can’t pick just one.
Don’t Take Me Alive by Steely Dan, or Heavy Fuel by Dire Straits
Jukebox Hero – Foreigner.