Canon's most versatile macro flash system is the Macro Twin Lite MT-24EX. It's comprised of two flash heads attached via coiled cords to a control unit mounted on the camera's flash shoe. The cords allow independent positioning of each of the flash heads on a bracket attached to the front of the lens. Some Canon macro lenses actually have the adapter for the flashes built into the front of the lens for easy attachment.
I use this system extensively for field and studio work. If you're looking for a more compact system, Canon's MR-14EX is similar to a ring flash, but the lighting ratio on each side of the circle can be controlled individually to give a less flat look to the light. For more advanced applications, you can control additional Canon flashes wirelessly from both the MR-14EX and the MT-24EX.
A handheld Canon EOS 5D Mark II, an EF 100mm macro lens and two small flashes on a bracket (MT-24EX) comprise a system that allows me the flexibility to keep the lens parallel to this rainbow boa's skin for efficient use of depth of field, even as the snake moves. The two flashes, set at different ratios, generate a natural-appearing light that brings out the details of the scales.
The Nikon Wireless Close-Up Speedlight System attaches to the front of the lens via an adapter, and control over the Nikon flashes is totally wireless. The flashes can be controlled via the built-in pop-up flash on some Nikon camera bodies or by an optional Nikon SU-800 Speedlight Commander, which sits on the hot-shoe of the camera. The Commander allows more versatility in controlling the individual SB-R200 flash heads that are the wireless component of the system. In addition to the two flashes held near the lens, you can position as many of these small flash heads as you wish around the subject without any cords getting in the way.
The Olympus Twin Flash System FS-STF22 features two flash heads attached to a bracket that mounts on the lens or lens hood. The Macro Flash Controller FS-FC1 sits on the camera's hot-shoe and is connected to the flash heads by coiled cords. Sony offers the Macro Twin Flash Kit HVL-MT24AM, with a controller, a circular bracket and two arms that hold the flash heads in various configurations.