The Z80 is actually still used and produced (belive it or not). And come in forms that can very greatly, but rest assured they are forms that are faster then 4 MHz. The Z80 was used in many home computers and consoles as evilandi mentioned (Including: Gameboy, Sega Master System, NeoGeo, etc) - it was also used in just about every pice of arcade hardware from the 80's till now (Including the renound CPS & CPS2 systems, Segas System 8, System 16, and System 24, SNK's MVS NeoGeo system, etc, etc, etc....).

The Z80 is used quite alot with sound processing, for example QSound based boards almost always (I say almost as I'm sure theres an exception, but I can think of it) use a Z80.