While the reservoir may not be the most glamorous of liquid cooling system components, there are different designs to consider when trying to choose the right one for your PC.
Of all the various components of a liquid cooling system that all work together to keep your computer icy cold, the one that makes the least difference is the reservoir. In fact, I've seen a custom liquid cooled PC that used the bottle the fluid came in place of a reservoir. It's true - reservoirs don't have to be anything special or fancy to do their job, but the one you choose can have a significant impact on the rest of the system. While you don't have to pick out a special reservoir to suit your system, doing so can definitely have some snazzy benefits. We'll talk about the different reservoirs you can find and how they can benefit your custom PC.
The most basic and often least aesthetically pleasing reservoir design tends to be what I call the "tank" reservoir. With this method, the reservoir is set or casually "mounted" inside or outside the chassis, and serves no other purpose but to simply hold fluid that is not currently being circulated. Is there a problem with this method? None whatsoever, but it also doesn't add anything to the system. Additionally, you may find it inconvenient - depending on your configuration - because it usually takes up more space than a purposefully designed one. Examples of a tank reservoir are the Thermaltake one pictured (picture also shows the connected pump), the bottle your fluid came in, or the slightly more stylized "tube" design found here.
Drive Bay Reservoirs
It's true that most full tower chassis' come with more 5 1/4 inch drive bay slots than the average user knows what to do with. Of course, you may be the exception, with every kind of optical drive under the sun installed, but most make due with a DVD burner combo drive, and maybe a Blu-Ray. The point is, that most of us have at least one drive bay slot open at all times, so why not slim down the space needed for your liquid cooling system by using that bay for a reservoir? Not only is it a wise and convenient use of space, but it looks pretty awesome as well.
Drive bay reservoirs come in single and double height configurations. Some also house other components, like the pump. You can get drive bay reservoirs that include fluid level indicators, temperature gauges, and flow meters, or you can get a very simple one that does nothing more or less than its job.
While the name is a trademark referring to a $250+ liquid cooling system made by Zalman, in casual conversation the name has been adapted to refer to any reservoir that also acts as a radiator. Most of these are external passive cooling devices, like the aforementioned and pictured Zalman. The design is actually quite logical: Instead of plumbing hot fluid through a radiator and then into a tank to do nothing more than sit, they skip a set and create a device that lets the fluid sit, and cools it at the same time. Just keep in mind that if you opt for a reserator that it will either sit or be mounted outside of your chassis.