The C128 also sports a Z-80 processor in order to run CP/M. In fact, the Z-80 plays such an important role in the C128's boot sequence, that the machine won't function if you remove it.. :-) Update: The Z80 checks for the C= key and cartridges in the Expansion port and triggers c64 mode automatically if a cartridge needs it.
It is possible to use the Z-80 as a coprocessor at the same time one's running normal 6502 code, but no one has come up with a useful application for this..
The C128 also has an 80 column VDC display that requires an RGBI monitor to work. You can see it in monochrome with a normal composite monitor, but it's too blurred to be of use. The VDC display really helps productivity work and terminal emulation.
The C128 is still the best 8-bitter CBM managed to develop, and it still has an active user base. People use CMD hard drives, accelerators and memory cards to make the machines do most things people do with their Wintel PCs.
Recently Maurice Randall completed his The Wave web browser that runs under the Wheels upgrade for Geos 2.0. Just think, graphical web browsing on the C128!