I'd like to consider this an evolving article. If anyone knows of any particular examples not listed here, mention them int he commetns section and I will add them with appropriate references. At any rate, here goes...
Conrad Veidt as The Joker (thanks to aknightinarkham)
Yep, this actor's prefromance in the film
The Man Who Laughs was the original inspiration for the Joker's appearance.
Fred McMurray as Captain Marvel
Although never officially confirmed, numerous texts lay claim to Captain Marvel's appearance being based on Fred MacMurray.
Whether or not it's true, to my eyes, Alex Ross definitely used him as a template.
Paul Newman as Hal Jordan
Although Errol Flynn has been assumed, the more dependable word seems to indicate that artist Gil Kane based Hal Jordan's appearance on Paul Newman.
David Niven as Sinestro
Similar word has it that David Niven was the baseis for Sinestro.
Dick Van Dyke as Elongated Man & Mary Tyler Moore as Sue Dibny (thanks to DexterDouglas)
Yep, apparently these two characters were originally created to resemble the famous TV couple.
Steve Reeves as Hercules (thanks to GsizeManThing
The actor's portrayal of the son of Zeus on the big screen seems to be the clear inspiration for the Marvel Comics superhero version of the character.
Various Celebrities as The Hellfire Club (thanks to aknightinarkham)
Writer Chris Claremond and artist John Byrne created pretty much drew every member of this evil organization with an appearance based on someone who they're named after.
Robert Shaw as Sebastian Shaw
Donald Sutherland as Donald Pierce
Orson Welles as Harry Leland
I assume this is an in-joke referring to his famous role as Harry Lime in
The Third Man.
Diana Rigg as Emma Frost
Another in-joke, regarding her role as Emma Peel on The Avengers.
Peter Wyngarde as Jason Wyngarde
Mastermind had been previously created, but it was with the Hellfire Club that he got a name, and his appareance was based on Wyngarde, specifically as his appearance in the popular British television series "Jason King."
Gregory Peck as Batman
In the classic
Batman: Year One, artist David Mazzuchelli based Brue Wayne's appearance, rather unabashadly, on Gregory Peck. This is where it starts to be a case of the artist making his own fantasy-casting. I can just picture Miller & Mazzuchelli in meetings together, going "Dude, you know who would've made a great Batman? Gregory Peck!" "Well, let's do it, then!"
Once again, this was continued by Alex Ross in
Kingdom Come.
Samuel L. Jackson as (Ultimate) Nick Fury
Then, of course, there's this classic example from artist Mike Allred,a nd one that actually became a reality!
Paul Giamatti as (Ultimate) The Tinkerer (thanks to DarkKnight2149)
This one came from artist Mike Bagley, under Brian Michael Bendis' instructions.
Brad Pitt as Nemesis & George Clooney as Sarge Steel
Artist Gary Frank drew these characters to resemble the two actors quite obviously in
Wonder Woman Annual Annual #1 in 2007. Very difficult to find pictures, though.
Christopher Reeve as Superman and Marlon Brando as Jor-El
Obviously a big fan of the 1978 movie, Gary Frank also drew these characters as their respective players in that film in
Superman: Secret Origin (albeit, the latter with an added beard).
Josh Holloway as Tony Stark (thanks to Ruadh)
In 2010, artist Salvador Larroca made his casting choice pretty clear by drawing the character pretty much identical to the "Lost" star.
Andrew Garfield as Spider-Man
A couple of years ago, Spidey had a cameo in Captain Marvel's comic, where he was drawn as Andrew Garfield. Apparently, the artist was fond of that casting.
Know any others? Please share!