Six Flags Great America

The best deal right now is to buy season passes for next year. I believe these are also good for Fright Fest this year. Check their website. If you buy four season passes at 64.99 each you get a free parking pass as well. This is a GREAT deal. Parking alone is $20 each visit. With the parking pass you can leave and eat outside the park and come back in the same day. We got modified season passes this year (can't use for Fright Fest) and had a lot of fun going.

Search Craigslist for cheap Six Flags tickets. I just did a quick search and there are quite a few people selling them. Six Flags was handing out free tickets to anyone who went during a power outage in August, so those are probably the tickets being sold. We have season passes, so I sold the ones we received. They are good thru the end of October.

I did notice someone selling tickets that are good for today and tomorrow only. It's closed this weekend, but maybe there is a private event. I would be cautious about buying those tickets. Ask why they are good for just this weekend.

I know the season pass is a really good deal if you plan to go more than once.
And Coke used to have discounts on their cans...not sure if that's still the case, but it was something like $5 off of each ticket at admission...

If you know anyone with an entertainment coupon book, the ones you buy around this time of year. The 2011 one had coupon for 15.00 per admission. However if you plan on going next year the season pass deal that includes this years frightfest is the absolute best deal and in 2 visits per pass purchased you will have fully have gotten the value of the pass. You do have to purchase 4. With the water park included plus parking this is usally the absolutely best deal unless you plan on a one time trip for a long time. When we had our season passes we went 17 times that year and didn' t spend money on really anything else that summer as we packed our own meals and ate at the picnic tables in the lot and snacked out there as well and felt okay to leave when the first person showed signs of being tired instead of trying to make it worth the day admission and stay until everyone was tired and grumpy.
Oh and just saw one for 4 tickets for like 110 or so if you use your Discover card. Must use by Oct 11.

their website is usually the best place honestly