The Secret of Monkey Island (first game)
Monkey Island 2: Lechuck's Revenge
The Curse of Monkey Island
Escape from Monkey Island

The first two games out of print and very hard to find...on ebay they go for around $50 or so..
Or you can *cough* download them *cough* from P2P or something.
The other two games are pretty easy to find.
I know PC Gamer magazine about 4-5 years ago released a CD with one of their print issues that had the full version of Monkey Island 1 on it.