Rodan (1956): The second kaiju film by legendary Godzilla director Ishiro Honda introduces the classic monster of the title, a giant pterodactyl bent on destruction who would become a mainstay of Japanese monster cinema.
Monsters (2010): This critically acclaimed low-budget hit is not only a nice primer for Pacific Rim, but also a good chance to get acquainted with the work of Gareth Edwards, who's releasing his Godzilla reboot next year.
Terror of Mechagodzilla (1975): Another alien-invasion kaiju flick, this film pits Godzilla against both Mechagodzilla and Titanosaurus for some of the most memorable fight scenes the King of the Monsters has ever been through.
Hellboy (2004): OK, so it's not a full-on monster movie, but it is one of Guillermo del Toro's major films, and the third act features some pretty huge cosmic beasts. It's definitely worth revisiting before Pacific Rim.