Here is yet another area of great debate.  Which are the best and which are the worst?  Well I use Dolphin pumps on my main tank, MarineLand power filters on my fresh water tank.  Since the size of my other tanks are so small I really have no use for pumps with them.

    I chose Dolphin pumps because they seemed to have the benefits of in sump or in line use, used lower power than most, and pushed enough water for my needs.  I also could not find much negative information on the net about them save some comments about the manufacturer working to correct a seal problem on a model I wasn't going to use anyway.  The price was about mid level, so I bought them.  They have been fine pumps so far and I have not had any trouble with them.  For this reason I could recommend them.
Update 2005:
Bad Pump... -- Coral Killer!  120v straight to water... yikes!  I think I'll try Eheim for main pumps next, the little one I have seems rock solid.

There are others that are commonly used that simply should not be used at all.  I will not name names, but a quick search on the net for information will give you a good idea of those.

    The basic things to look for include low power consumption per volume of water moved, low heat dissipation, the required flow rate, longevity and serviceability.  Ability to handle back pressure or not is one area you will want to pay attention to.  If your planned use for a pump involves back presure you will need to get one that can deal with it as those that are not built for it would not last too long.  Other than those issues you can choose any pump that you like.

    One warning though.  That is that as with many other areas, you get what you pay for most of the time.  Buy crap and you get crap.  By the same token, if you only settle for top of the line you are likely getting too much for your needs at a raised cost.