Bunkie boards are thin pieces of plywood covered with material, usually something that is flame-retardant, that lie beneath your bed on the ground. They are professionally cut down to fit the size of a mattress. Often, the same material that is used to cover a mattress is used to cover a bunkie board. They are generally 3 inches or thinner so that they provide support without taking up as much space as a box spring. Bunkie boards are ideally suited for bunk beds or beds for which vertical space is an issue.
A link spring is a grid-like apparatus that uses springs to provide support. It is made of metal and is durable. Simply lay your mattress across the link spring. Common bed types that use link springs are daybeds and trundle beds.
A platform bed is a bed frame in which the mattress rests on the bed frame instead of a box spring. Platform beds have become increasingly popular, and some are less expensive than a box spring. Some have an under-bed storage area, like a regular box spring-mattress combo does, while others use that space for drawers or shelving. A water bed is a type of platform bed, although most platform bed frames do not surround your entire mattress like a water bed.
Better Than a Boxspring
The Sleep Revolution company makes a product called Better than a Boxspring. It is an all-in-one item that combines both the traditional metal frame and box spring. In doing so, it saves space and prevents having to make two purchases. It provides space underneath for storage, as a box spring does, and Sleep Revolution claims it can cut down on the mites that sometimes hide inside traditional box springs.
Plywood and Pallets
If you have a bed that is sagging but don't want an entire boxspring -- or if the box spring sags, which is a possibility -- then an alternative is plywood. A thin, 3/4-inch sheet of plywood will help support the mattress and keep it from sagging without the bulk of a box spring. Large shipping pallets like the ones you see outside of grocery stores and warehouses also work, though you may need more than one depending on how big your mattress is.