Well, officially the free parking is only for Dorney Park Season Pass Holders... although it doesn't hurt to ask, and I'm sure there are some employees who would let you in with another CF park's pass.
I was at Knotts last April and I didn't get to park for free with my CP parking pass. The employees at parking weren't sure if I could or not. They charged me and said to go to the general sales (?) office and they could give me my money back if I could park for free. Didn't work.
It's really cool when a sisterpark parking season passholder KNOWS that they shouldn't be getting free parking at a park other than their own, but tries anyway. Parks love that. So do the cars behind you in line for the parking booths. And the people you complain to at the front gates.
It's not worth it people. For everyone's sake, start your visit off right...cough up the money and enjoy the rest of your day :) :)
Dannerman the parking pass was for Cedar Point. Knott's was the first other Cedar Fair park I tried it at and the attendant didn't blink an eye and let me park free. A few months later I tried at Michigan's Adventure and they laughed. ST chick, I'm not the complaining type.:)
Perhaps another option CF could look into is offering reduced parking for sister parks passes/employees. I'm sure it wouldn't really hurt them much to offer passholders parking for like $2 - but that silly little 'perceived value' thing would skyrocket.. heck, even for $5 I'm sure there'd be a lot of buzz among the season pass community.