You need
Opposing Force, Blue Shift and
Decay!
They're all separate games and there's space in your poll.
Opposing Force is my favourite.
Mainly because the weapons in it are completely mental.
There's an electric bug you steal from the corpse of an enemy, a Squid Bazooka, a Machine Gun so powerful it makes you move backwards when you fire it, a Deagle with a laser sight, Snarks, a Barnacle (yes, you get a barnacle to use as a weapon) and the original version of the Portal gun.
The portal gun in particular is brilliant, using it's secondary fire opens a portal to a random destination, sometimes you get something good, othertimes you immediately fall to your death.
The actual gameplay is much the same as Half Life, but your tag along buddies are more effective this time, did I mention the brilliant weapons?
Maraveno said:
But that's Portal, if it were Half Life it would be called
Half Life: Portal, not just Portal. Think about it...