According to Civil Avionics Systems, such system is found on the Airbus A350, A380, Boeing 787, and new business jets. But only certain systems use the variable frequency (VF) AC. Namely motors such as in the fuel and hydraulic systems. They are designed to operate at different speeds with tolerances allowed for the lowest mode of operations.
In short, the motors, heaters, etc., are designed to operate on different loads reliably.
From the 787 manual, items such as cockpit displays and instruments use DC power instead.
Snippet from the 787 manual.
The above VF system is by far the lightest and less complicated. Between it and the IDG system, there was the short lived variable-speed constant-frequency (VSCF) used on the McDonnell Douglas MD-90. In its early days it proved troublesome to maintain but eventually it achieved high dispatch reliability. As with the VF, the engine generators outputted VF, but then tap converters were used to produce constant frequency. Below is the MD-90's air-scoop for the cooling of this system.
(airliners.net) Air-scoop for cooling the VSCF on the MD-90.