I've got a Little Giant 2-MDQ-SC, but I'm opting to use two Johnson pumps instead:
http://forums.procooling.com/vbb/sho...&threadid=6650
Neither of these are "normal" pumps though (they have thier advantages and disadvantages).
The popular choices are: Eheim (1048, 1250), Via, Danner, Maxijet, and even Rio, among a flock of others. Top of the line would be Iwaki.
The Eheims are the most expensive, out of the aquarium field, but run quiet and are very reliable. If you look into industrial products, you'll find a whole lot of units (like March), but you really only have to watch out for a couple of things:
1-Be aware if the pump is self priming or not. Some cannot be run dry, under any circumstances.
2-Be aware of the noise they make: centrifugal pumps are usually the best choice here.
3-Make sure that they are rated for continous use. Avoid bilge pumps.
4- Understand that pumps are spec'ed with a max head, and a max flow rating, which are two seperate things.
5-Be aware how these must be mounted: some centrifugal pumps do not like to be mounted with the motor down.
6-Mke sure that you know what the connections are. The Johnson pump has 3/4" barbs. My Little Giant has 1/2" NPT threads.
Once you've selected a pump, go over pHaestus' article, and find the part about inlet pressure (a link to a PDF, I believe), and note how the inlet should be set up.
Good luck!
(Did I miss anything?)