.
Leonard Nemoy for Aleve:
Leonard is preparing for a Star Trek convention, but is on the phone with his agent insisting he needs to cancel because of the arthritis pain in his hand. His agent suggests he take some Aleve. He does, and, sure enough, his pain dissapates; and, much to the delight of the Trekkies, he's able to give the Vulcan peace sign.

You can view it here.
MacGyver for MasterCard:
You know the MasterCard commercials. Now they've made a beautiful one with our beloved Richard Dean Anderson as MacGyver. (Helen, have you seen this?) I can't even do it justice in description, so you've just got to watch it!

I know there's at least one more commercial I meant to include here, but my sieve of a brain can't recall it right now. I'll update it later.
EDIT:
Ah, yes...how could I forget the LOTR commercial for TBS?

Play it here. It's the last line that gets me.