Cracked, a site for anything humorous, has proclaimed that Pirates of the Caribbean was, indeed, a rip-off of Monkey Island. An excerpt:
The video game The Secret of Monkey Island came out in 1990 and follows the adventures of Guybrush Threepwood, a bumbling swashbuckler who must gather a crew of pirates to rescue the woman he loves while dealing with a mysterious supernatural curse. Sound familiar? That's also the plot of the first Pirates of the Caribbean movie, except Threepwood is called William Turner there. Both characters even dress alike.
Rip-off or not? Does anyone really care anymore? Either way, it's a fun little read anyway.
Source: Cracked