It always seems to be a guy, you know the one. He has a line like:

YOU REBEL SCUM! (The Empire Strikes Back)

Or suchlike. There's usually one in every film: someone who tries to hog the scene for about three seconds. Who are your favourites?

I hate to use the awesomest of people at the start. But it has to be the guy from Starship Troopers. He appears before but doesn't do anything, can't remember his name though. =P
Here it is anyway:
"Man, they really humped the bunk..."

There's a guy in 'Helen of Troy' - he doesn't actually have a line, but his "amazed" face when they bring in the wooden horse is amazing in itself...

I just thought of another one I love - the guy who shouts 'OICEBERG! ROIGHT AHEAD!' in Titanic. What a star!

Andy Peters in the UK release of Toy Story 2. He shouts something when they are putting the luggage onto the aeroplane (I remember watching Live And Kicking when he was at the Pixar studios. True story)

I remember that! I was gonna use it too damn you

ErmI suppose Stan Lee would have to rank up there, the obvious cameo's in most Marvel films (especially the newer ones) but also in Mall Rats... he's hilarious!

Stan Lee's cameos are great. I think I did see a vaguely subtle one once, but that was the exception. He seems to be drawn to roles as a security guard... I'm not quite sure why: would you be intimidated by Stan Lee chasing you? (Assuming you didn't actually know it was Stan Lee. If you did then you'd have every right to be terrified, because really he leaps between rooftops at night, blinding evil with the magical lens in his glasses.)