Metal Gear Solid: Portable Ops (commonly abbreviated to MPO) is a stealth game directed by Masahiro Yamamoto, with series creator Hideo Kojima acting as a producer. Portable Ops was developed by Kojima Productions and published by Konami in 2006 for the PlayStation Portable. It is the third Metal Gear title for the PlayStation Portable and the first one to retain the series' action-based gameplay. It is also the first canonical outing of the series for a portable platform, being a direct sequel to Metal Gear Solid 3: Snake Eater.

On July 17th, Konami announced Metal Gear Solid: Portable Ops Plus, an expansion of the original game focused on online play. Among its new features, the expansion includes new characters, missions, tutorials for beginners, as well as a new single-player mode called "Infinity Mission". In Japan, Portable Ops Plus was released on September 20 in two formats: the expansion on its own and as a deluxe package that includes the original game and the expansion. The North American version was released on November 13, 2007. The European release date has not been officially announced, however, Konami have said that it will be available in PAL territories in spring 2008.

Here's a useful list of passwords and unlockables in case you ever get good old Snake into trouble:

Passwords

Unlockable Character Passwords
Enter the password in the "Recruit" "Password" section of the main ingame menu.

YLFSSVBCDLGO - Dengeki Playstation Magazine Soldier

ZKUJZUYXJUH - Famitsu Magazine Soldier

TTOGZWSREYD - Unlock Arms Magazine Soldier

Unlockables

Unlock special characters.
Beat the different modes on infinity mission to unlock different characters.

Naked Snake - Beat easy mode in infinity mission.

Old Snake - Beat Medium mode in infinity mission.

Raiden - Beat hard mode in infinity mission.

Roy Campbell - Beat extreme in infinity mission.

Snake strikes back on the PSP: