Often, these "n00b cannons" have heavy disadvantages, and the guy who keeps getting killed is probably doing something horribly wrong.
Regardless, use of these weapons in public multiplayer will get you mocked and shunned, even though they are valid weapons, with only a couple exceptions.
The Auto Shotgun in Counter Strike.
The Grub from Gunbound could possibly be considered a n00b cannon.
The Shotgun in Natural Selection.
The SPAS in Navy Seals-Covert Operations.
The Chaingun in Team Fortress
The OICW in SWAT 3.
The Panzerfaust in Enemy Territory. (Restricted on some servers)
The Jackhammer in Planetside.
See any patterns in there? Lots of fast-firing shotguns. Anyone ever hear of sniper rifles to counter?
Although not limited to FPS and MMORPG genres, it is primarily used as a sarcastic remark against anybody who gets 'lucky kills', or just happens to be in the right place at the right time (Specifically against people who otherwise show little-no ability in the game).
Grenade launchers in general. (Battlefield, GunZ)
Tanks or heavily armored vehicles. (Battlefield, Halo)
Automatic shotguns. (CS:S, GunZ)
Machine guns. (CS:S, GunZ)
Semi-automatic sniper rifles. (CS:S)
Any mid-short range weapon that has a high probability to kill in one hit. (Any/all FPS)
Assassin or Sniper abilities (courtesy CoZ)
Low-level characters with powerful magic or weapons (ROSE, Runescape, WoW)
'It was the guy with absolutely no score. Well.. He has a score now, anyway.'
"Lucky n00b! I'ma kick your @$$ next round!"