The Tech 21 California is pretty good. I A/B'd it with my Mark III (and on my Mark III) and it gets within about 90-92%) of the Mark sound. Its actually super nice, especially for the early woody clean sounds. On the super saturated high gain stuff, its just a little too grainy. Nothing really cuts like a IIC+ though. The California's highest gain setting emulates a recto though, not a Mark, so it would sound closer to a DC.
Yeah, I'll second the Tech 21 pedal. In fact, most of the Sansamp pedals (I've got an old GT2 that I carry in my gig bag anytime I have to use a rented backline) can do a convincing Mesa tone when set to "California".
