Unreal Tournament has 5 different turrets, featured in the Assault and Onslaught gametypes. Turrets are stationary, but provide some heavy firepower. Being stationary, Turrets can be easily destroyed, but provide an excellent defensive barrier.
The Ion Cannon normally only features in the map AS-RobotFactory. It's a huge pink/purpleish cannon with a similar primary fire to the Ion Tank if a little bit bigger blast radius. It's quite well armoured too, so it takes a bit of punishment. The obvious disadvantage is that it's no good at close range. An award is given for destroying one.
The Energy Turret is found in Onslaught Game Modes. It almost always accompanies bases and most PowerNodes in Onslaught games. It's quite strong, and is effective at medium to long range (When inside the turret, by pressing the alternate fire button (usually the right mouse button), it zooms in on the area you have the crosshairs fixed on. To zoom back out simply press the alternate fire button again) with the primary fire being powerful energy beams. If the user is good at aiming, these can serve as a useful tool for taking out other vehicles and players alike, especially Mantas and Raptors which may be too fast for other weapons to hit (discounting the AVRiL). These turrets are very useful in shooting down any enemy aircraft that may be in the area.