It all depends on stocking.
The most common method is the Berlin system consisting of Live Rock, powerheads and a skimmer, as detailed above.
If you want a higher bioload then a wet dry becomes very useful. Fluidised sand beds are often incorporated into massiove systems as they hold a massive amount of surface area in small package.
An infrequent but very good system is the algae turf scrubber. This uses algae to filter the water and bypass the nitrogen cycle and use up other polutants such as phosphates.
The main reason people tend to stick with LR and skimmers is it is fairly easy and low maintenance. Mechnaical filtration will certainly benefit any system, but you have to do maintenance on the media often (at leas once a week) to prevent the filtered detritus breaking down and causing a nitrate spike.
If you don't mind buying lots of salt then you can just use a standard "FW" filter and perform weekly maintenance and a pretty heavy water change once a week.
It's all down to what you want to keep, and how often you want to maintain.
