Enzotech CFS-ID3/8-OD1/2 Water Cooling Accessory

Pros: Inner Diameter: 3/8"; Outer Diameter: 1/2" – Just what I needed!
Bench tested with no leaks, and no leaks a month or so later.
Good quality -- No sharp edges that may cut through hoses.

Cons: If you use older "Ballooned" hose, you may have to file the cap a bit to slide onto the hose. I think that voids any warranty though.

Pros: I got a few of these for the water cooling setup in my new PC. The ONLY tubing that will work with these is Inner Diameter: 3/8"; Outer Diameter: 1/2" (Wall is 1/8" [.125"]).
As long as you are using this kind, there will be no leaks whatsoever. Make sure you push the tubing all the way onto the valve, and then screw on the nut as tightly as you possibly can, to prevet leaks.

Cons: A little heavy, but I don't really care that much about that.

Other Thoughts: No leaks, but MAKE SURE you test all components in a watercooling setup for several hours BEFORE putting it near sensitive electronics

Pros: Made a very tight seal with Tygon R-3400. Make sure you match both ID and OD to your tubing. No leaks.

Cons: None really.