Dude, you knew I was going to rec. the Rena XP3. I bought a used one from craigslist and it works great. I was originally going to get the Marineland (Tetratec) but the specs were better on the Rena as far as flow is concerned. I've also heard a few horror stories about the tiny gaskets being a pain between the media baskets in the Tetratec/Marineland canisters. Never ended up getting one of those so I have no idea if that's a real concern or not.
Oh, and Zach's 55 gallon is a bit understocked, but he runs an XP2 on that tank which we bought new from Amazon. The XP3 will be that much better.
Credit to FishFanatic04, Oldman & AlmostAwesome for convincing me that the Rena Filstars were the way to go.