Not people but cool instrumentation that makes a brief appearance in a completely appropriate way. It always impresses me when an artist will use an instrument sparingly in interesting places instead of going overboard because "we've hired this sax player - let's throw him all over the whole song."

Off the top of my head this AM:

Walkmen - "Louisiana" - The piano slide thing into the mariachi shit is stellar, obviously.

Whigs - "Can't Hear You Coming" - Also has some horn lines that pop in at the very, very end to great effect.

REM - "World Leader Pretend" - It's got that pedal steel just on the bridge that gives me shivers every time.

Any others? Or you can just post pics of Camaros.

Beatles - "For No One" (french horn)

Best cameo of a cockplate.

There's some cool French Horn in After the Gold Rush.

There could be a whole other thread about songs that tag the bouncy, stately french horn bit to the end of songs to really hammer home the Beatles homage.

"Cary's Here" - Hop On Pop
The inclusion of the trumpet solo was directly inspired by "For No One"