In this list I'll only include instances where I've seen the whole movie or play in question, not just YouTube clips. For example, I know Hugh Jackman has sung on stage many a time, but I've never seen one of his performances, so I can't really say I've formed an impression there.
However, I will provide YouTube clips for each of these men so you can enjoy them yourselves.
1. Ewan McGregor, Moulin Rouge. Ewan won me over as a lifelong fangirl through this film. The tragic love story and glittery sets would have been enough, but the heartfelt, skilled singing clinched it.
2. Neil Patrick Harris, Dr Horrible. Neil's a Broadway actor and singer, too, but this is the first time I was treated to his talents. So sweet, so funny!
3. Billy Boyd, The Return of the King. The Lord of the Rings films were full of great music, but Billy's a cappella gave me the biggest goosebumps.
4. James Marsters, Buffy the Vampire Slayer. More Whedonverse here. God love the man for encouraging singing. Not only is James, as Spike, witty and sexy, but he can sing a mean Goth-rock, too.
5. Anthony Stewart Head, Buffy the Vampire Slayer. And while we're on "Once More With Feeling," I can't omit Giles and his golden English voice. *sigh*
6. John Cameron Mitchell, Hedwig and the Angry Inch. He's adorable in drag, and the movie is ten times sweeter and more clever than you expect going in. Plus, could the music rock more? I thought not.
7. Guy Pearce, The Adventures of Priscilla, Queen of the Desert. For the most part, Guy Pearce, Hugo Weaving, and Terrence Stamp don't do any actual singing in Priscilla, just lip-synching. But they're so fabulous that I had to find a way to include the film. So here's a clip of Guy singing for a few seconds. (Warning: some cheeky profanity in this one.)
8. Rick Moranis, Little Shop of Horrors. Reaching back to the 80s for this one, but what the heck. I sure hope Rick's gotten a chance to sing more than this one part, because he totally rocked it.
9. Tim Curry, The Rocky Horror Picture Show. Reaching back even further for this one, but how can I not include Tim Curry in Rocky Horror? I rather doubt anyone else ever took camp so far, so successfully.
10. Gerard Butler, The Phantom of the Opera. Okay, not everyone can live up to Michael Creighton, but Gerard did a fine job trying, and is a fine man as well. Way too good-looking for the Phantom, actually.
It occurs to me that I could go on way too long with this list if I wanted, so I'll stop at ten. And I'm sure I'm forgetting someone obvious, so remind me.