Here's an interesting device from Thermaltake. Some fears people have with water cooling are leaks developing in the system and poor heat transfer/water flow. It's not always easy to detect a leak and it is difficult to measure the actual efficiency of your cooler over time, especially without an accurate tool. This nifty device from Thermaltake, which resides primarily in a spare 5 1/4" bay, alleviates this. It allows you to read the water level in the system, the speed at which the water is flowing and the temperature of the water (or whatever cooling liquid you use).
It can be snapped in to most existing water block setups and looks quite fancy. There's a review of it here at rbmods.