There are few things in life greater than Half-Life 2 and Portal's turrets, but combining the two together might just cause the universe to implode. Enter Entropy: Zero 2, a fantastic new total conversion from modder Breadmen, which sees the player take up the role of the Combine. That's right, you're the bad guy, and you're after one man, Gordon Freeman.
You'll revisit some familiar locales including Nova Prospekt, before tracking down Dr. Mossman in a snow-covered facility - which presumably all ties into the events of Half-Life 2. From what I have played so far though, it's pretty fantastic stuff - featuring a fully voice-acted, detailed story campaign that should bring around 10 hours of gameplay.
And of course, there's the turret from Portal. Given both games exist within the same canonical universe, it's totally plausible, and brings some incredibly wacky moments to the story.
The turret is named Wilson (presumably a reference to the Tom Hanks classic Castaway) and you can use him to get the upper hand in a number of situations you'll find yourself in as you progress.
In addition, the mod features a number of brand new enemies, weapons, and environments in addition to familiar ones. You're even able to give orders to teams of Combine, and deploy manhacks to track down and kill your enemies.
If you're interested in playing it, and I recommend that you do, you can grab it now on Steam.