They're corner filters or box filters and they used to be the only filters available. The AquaClear MINI is another possibility since the flow rate is adjustable and you'd probably keep it at the lowest setting for a 5 gallon tank.
Here's a page I found that has pictures of the box/corner filters we're talking about (scroll down the page a bit):
http://www.dalecombp.com/filters/internal.html
One disadvantage of these filters in a small tank is the fact that they take up interior space. The AquaClear Mini would just hang on the back of the tank. When you siphon out old water during partial water changes, rinse the filter sponge in the old water to preserve the beneficial bacteria. To save money, I buy the charcoal in bulk and use reusable media bags.
To maintain beneficial bacteria, I don't rinse the sponge when I change the charcoal media. So I alternate when doing water changes; I rinse the filter sponge one time and then the next time, I change the charcoal. The reusable media bags are great money savers and I actually like the AquaClear filter better than other power filters I've used because of the reusable filter sponge and ability to use your own media instead of buying expensive cartridges.