There are several options out there for you. You can go the HOB (hang on back) way, or the canister way.
For the HOB route, aquaclears can easily do the trick. One aquaclear 500 with two sponges (as opposed to a sponge and a carbon bag) can handle a moderately stocked 55 gallon. If you want to go the safer route, two aquaclear 300 both stuffed with two sponges can easily handle anything that a 55 gallon can contain (within reason).
Emporers will also work, but I've had very limited experience with these filters, so hopefully someone else will come in on this one.
As for the canister route, a fluval 404 should do the trick. As will a proper Eheim canister. If you want the top filter, go Eheim. They are very quiet, extremely realiable and imho better built than the fluvals. I own both filters, both have worked for me for quite awhile.
Eheims - extremely good biological filtration, lacking somewhat in mechanical filtration.
Magnum canisters - good mechanial filtration, lacking somewhat in biological filtration
Fluvals - somewhere in the middle.
I heard decent things about filstar filters as well, hopefully someoen else can fill you in on those filters.
HTH
-Richer
