The third game released in the Metal Gear series was developed and published by Konami in September of 1998 for the Playstation, and again in 2000 for the PC. It was the first game in the series after eight years, and was highly anticipated at the time. It was later remade in 2004 as Metal Gear Solid: The Twin Snakes, is followed by a sequel (Metal Gear Solid 2), and a prequel (Metal Gear: Ghost Babel).
It was also the first game in the series in 3D. Now, 1998 was the height of that particular period in gaming history where all the major video game series were making the jump to 3D (Zelda, Mario, and others all did this around this time, although Mario was slightly earlier), and MGS was one of the big ones. Final Fantasy VII had cemented the PSX's success about a year and a half earlier (and was, amazingly enough, the first 3D game in yet another series), so calling MGS the PSX's killer app might not be quite right, but it was amazingly successful and certainly didn't hurt.
It is a common thread with many of these series that their first 3D game executes the addition of the third dimension nigh-on-flawlessly (the aforementioned Zelda, Mario, and Final Fantasy, of course, and the more recent Grand Theft Auto III and Metroid Prime spring to mind as well), and MGS is no exception. The camera angle, always an important choice for any third-person perspective game, tries to emulate the top-down view of the previous, sprite-driven games, while also not being afraid to let the player peek around corners or look at things in the first-person. This is one of the more successful approaches to third-person camera angles, and is a refreshing change from the slightly more standard above-and-behind view.
Most of the game revolves around sneaking around. This usually just means staying out of the line of sight of the enemy troops, although other factors come into play as well. The biggest of these is sound: enemies can hear your footsteps, you can lure enemies around by banging on walls, and gunfire will usually just set off the alarms immediately. Enemies can also follow footsteps left in the snow (for this is Alaska, and things are a little cold out). So you (as Solid Snake) must evade troops, walk slowly on loud metal grates, and suppress your gunfire when possible.
Beating the game will unlock additional features, of varying usefulness; this makes the game good for a couple play-throughs.
The plot of the game has been detailed elsewhere, so I will refrain from talking about it here.
Highly recommended. Easily one of the top two or three best games for the Playstation.