Roller skating, like skateboarding, has created a number of spin-off sports and sports devices. In addition to rolelerblades/inline skates.
Normal-looking street/skate shoes with a concave plastic plate in the sole to allow grinds.
Normal-looking street/skate shoes with a single retractable wheel in the heel of each shoe, allowing the wearer to perform unique rollerskating-like moves at leisure while still walking normally when the skating functionality isn't desired (and the wheel is mostly retracted into a recessed slot in the heel). The fact that skateboarding and related wheeled sports are outlawed in many cities and suburbs makes the low key and spontaneous nature of Healys all the more enticing to the same demographic. Heelys were later also combined with Soaps into a single hybrid shoe.
Skates, a class of un-attached skates that wearers place under their normal street or skate shoes. They typically have 2 closely set inline wheels set with a short base under a small squarish plate (usually surfaced with grip tape about the same width width as the rider's shoe. This arrangement allows for a range of motion similar to single-wheeled skates like Healys. Due to the lack of straps on the contact plate, freeline skates require constant motion to stay on, and are a particular challenge for novices.
There are also other lesser seen two-wheeled skate arrangements. Some resemble inline skates but with 2 very large wheels bolted in at an angle from the outside rather than a center-balanced row of 4 smaller wheels underneath of inline skates. Others resemble freeline skates in that they have a small squarish platform, but with 2 medium-sized wheels on either side, somewhat between a freeline skate and roller skates (but with inline-skate-styled wheels).
Orbit Wheel Skates
Another spiritual relative of the freeline skate whereby the skate stands on a grip-tape-surfaced platform (just slightly larger than a freeline skate's) inside of a large hoop that contains a trapped wheel that can freely rotate under the grip plate each foot is planted on. The foot plates normally rest on the trough of the inner surface of these orbital wheels, with the toes pointing orthogonal to the rotation of the ringed wheel. It's said the experience of riding them is somewhat similar to skateboarding, and there are variants with the two wheels connected so the rider is fixed in a skateboarding-like stance.