Endless Pools are probably the most well-known type of lap pool replacement. A relatively small pool filled with water deep enough to swim in, they are "endless" because a propeller moves the water so the swimmer swims in place. This is a great form of exercise and takes up much less space than a regular pool, but it's not the only way to get these benefits.
The least expensive alternative, if you already have a swimming pool not large enough for real lap swimming, is a tether system consisting of a harness that holds the swimmer and a bracket or pole outside the pool that anchors the harness in place. The swimmer swims and doesn't go anywhere.
Portable Swim Current Generator
If you have a regular inground pool, you can install a portable swim current generator that mimics the action of the propeller in an Endless Pool. One brand of portable swim current generator is ExerSwim.
Build a Traditional Lap Pool
The dimensions of such a pool would be about 40 feet long and 10 feet wide. This still takes up less space than a full size pool, and has smaller operating costs, once built, than an Endless Pool.
Jet Propulsion Swim Spa
Also known as a jetted swim spa, this is basically a rectangular hot tub that has extra jets to provide moving water similar to the Endless Pool. Benefits include the fact that it doubles as a hot tub, complete with jets and fitted seats, and the small size: some models are even smaller than Endless Pools. Brands of this type of system include Aquatic Fitness System (AFS) and Hydroworx.
This pool has moving water powered by a paddlewheel instead of a propeller. A great benefit of this is the dimensions of the moving water. The Endless Pool's moving water is less than two feet wide and about 18 inches deep. The area of water moved by the paddlewheel style is as wide as the pool, and can be up to 30 inches deep. One brand of this type of pool is SwimEx.
This type of pool provides the option of walking against the resistance of the water, and taking advantage of the water's buoyancy, while still having the option to swim against a current. A brand of spa that has this option is HydroWorx.
Affordable Therapy Pool
This is the more affordable option: a soft-sided pool similar to those you may have seen in backyards or discount stores, except it is deeper and smaller. It is roughly the same size as an Endless Pool, but requires no digging and can be installed right next to a porch or even inside a garage. To get the swimming-in-place option, an inexpensive additional option similar to the tether is available. This pool has no moving parts and is significantly less expensive than an Endless Pool, but still costs several thousand dollars.