Pumps:
Pumps used in water cooling come in two basic varieties, AC and DC. AC pumps need to be plugged into a wall socket for power, while DC pumps can generally be run on 12V from the power supply.
This is a basic guide and I will not be covering AC pumps in it at this time.
DC pumps are often the best choice. Traditionally, they are smaller, quieter, cheaper, and do not add as much heat to the loop. (Motors, are not 100% efficient and the difference comes in the form of heat output.)
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Some of the more popular DC pumps include:
Laing DDC 3.2 (aka, Swiftech MCP355): This is a very small pump that offers excellent performance. It is one of the more expensive DC pumps out there. In it's stock form it has Twin 3/8” Diameter barbs molded onto the top that go out to the side. The nifty bit about this pump is that numerous companies make and sell “Tops” that take the place of the one that comes with the pump and that offer, among other things, improved performance, the use of standard metal barbs, and some models even have tanks that take the place of a separate reservoir. (See the below links for pictures of the stock pump and some of the more common “Tops”.)
*Note* This pump also comes in a low speed variant sold as the DDC 3.2 10W or the MCP350. This pump should be avoided unless silence is of the UTMOST importance. If you find a great deal on one, they can however be modified by soldering two points together to run at the normal 18W.*
Stock Pump:
http://www.acousticpc.com/images/a_l...p_lg_pic_2.jpg
Pump with After-market Top:
http://www.watercoolingshop.co.uk/im...-Laing-DDC.jpg
Pump with After-market Reservoir Top
http://www.chilledpc.co.uk/shop/imag...ddc-res2-l.jpg
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The second popular DC pump, also made by Laing is the D5 (aka, Swiftech MCP655 and 655-B): This pump is designed much differently and is much larger. It is universally regarded as one of the quietest and longest lasting pumps available. It also comes with plastic barbs molded into the housing, however these are the traditional 1/2” diameter that is often used in water cooling. Recently a couple companies have started offering “Tops” for this pump as well, which slightly improve performance, and allow the user to use standard metal barbs as well. This pump comes in 2 varieties, one with a built in Potentiometer, for pump speed adjustment, and another with a fixed speed. The fixed speed offers slightly lower performance than the adjustable version at it's fastest speed. The 2 are easily identified from each other as the fixed speed pump includes a RPM speed cable t monitor the speed of the impeller. This is the same cable that often comes with fans. The Fixed speed pump is also designated with a (-B) following the pump name. (See pictures below for pictures of the stock pumps and one with a “Top”)
Stock Adjustable Speed Pump:
http://specialtech.co.uk/spshop/file.../alc-13922.jpg
Stock Fixed Speed Pump:
http://us.st12.yimg.com/us.st.yimg.c...s_2044_3039539
Pump with After market EK “Top”:
http://us.st12.yimg.com/us.st.yimg.c..._2046_18113051
Both of the above pumps are held in well regard by water coolers the world over and offer excellent performance for money.
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For those on a tighter budget...
The Danger-Den CPX-Pro is a very small pump that offers the best bang for buck out there. There are no performance increasing “Tops” available for this pump, However, it does allow the use of standard metal barbs out of the box. If you choose to use this pump, you will be limited in the number of blocks and radiators you use as it's performance is much more affected by restriction.
Stock pump with metal barbs attached:
http://us.st12.yimg.com/us.st.yimg.c..._2045_43169624