Actors singing

Adam Sandler - Werewolves of London [originally by Warren Zevon]
Quite good. In fact, Sandler's mock gruff vocals seem to suit the song well.

Robbie Williams & Nicole Kidman - Somethin' Stupid
[originally by Frank and Nancy Sinatra]
Not bad, though a bit bland.

Jack Black - Let's Get it On [originally by Marvin Gaye]
I remember the first time I saw Jack Black was in an old episode of X-Files where he worked at an arcade (I think) with Giovanni Ribisi who could control lightning...or something like that. Or who could forget in The Jackal when Bruce Willis blew Jack's arm off with some sort of gatling gun. Well, he has since moved on to bigger and better things, and his scene-stealing performance in High Fidelity (from which this cover song is taken from) was a big factor in that.

Wakefield featuring Mary-Kate Olson - Suffragette City
[originally by David Bowie]
Half of the Olson twins plus some guy named Wakefield covering David Bowie should be a disaster, but it's surprisingly tolerable. Mary-Kate's contributions are thankfully limited to singing "hey man" and "suffragette city" every now and then.

Kevin Spacey - Beyond the Sea [originally by Bobby Darin]
I didn't catch the film, but judging from the soundtrack which is full of Kevin Spacey covering Bobby Darin tunes, he seems to do a pretty good job.

Bill Cosby - Sgt. Pepper's Lonely Hearts Club Band
[originally by The Beatles]
This cover is just downright painful.

Sarah Polley - Courage [originally by The Tragically Hip]
Completely different take on the Tragically Hip classic. From the soundtrack to the Atom Egoyan film The Sweet Hereafter, in which Polley also starred.

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Unlikely duets:
William Shatner & Henry Rollins - I Can't Get Behind That
From last year's Has Been, which of course featured the fantastic cover of Common People with Joe Jackson, this "song" is more of a back-and-forth rant between Shatner and Rollins.

Eddie Vedder & Susan Sarandon - Croon Spoon
This unlikely duet between the Oscar-winning actress and Pearl Jam frontman is made moreso unlikely by the fact that it's from a 1930's musical. Taken from the soundtrack to Cradle Will Rock, which was directed by Tim Robbins.