The 3-piece airlock is standard equipment in the brewer's arsenal. Made of 3 pieces (of course), this airlock can be disassembled and cleaned, unlike the traditional double bubble airlocks. Simply fill the airlock past the holes in the center piece with sanitizer or water. Because of their design, they have less of a chance of sucking liquid back into the fermenter than the traditional double bubble airlocks.
They can also be easily made into a blowoff tube by attaching some ½" ID tubing to the center of the main airlock body. You can never have too many airlocks!
These S-shaped airlocks are the classic design, used for years by winemakers. Fill them to the fill line and affix to your fermenter of choice. Positive pressure from fermentation will allow CO2 to escape through the water without allowing oxygen or contaminants into the vessel. These also make great airlocks for food fermentation or secondary fermentation of beer.