This may only be for the St. Louis Science Center (hence the YMMV), though you may want to check your local science center for something similar. ASTC

The Friends & Family annual 6-person membership plan is $85, and includes the following benefits:

• Free tickets to some exhibits (currently Ends of the Earth)
  
• 10 free OmniMax tickets ($62-80 value)
  
• 6 Free parking passes
  
• Free admission to the Planetarium, Discovery Room, and MINDSTORMS - Every Visit for up to six people per visit!
  
• Free advance reservations and Members-only ticket station
  
• 10% Discount in the ExploreStore
  
• Members-only film and exhibition preview events
  
• Many tickets are half price with membership, including OmniMax if you need more than 10.
  
• Reciprocal benefits at more than 250 science museums across the country
  

$85 Membership Benefits Details

Well, some of the perks I could care less about, though you may feel differently. I could care less about parking close, but for some this is a pretty big perk. Even with 10% off the store is too crazy expensive for me. The free tickets alone are worth it for me. While I wouldn't normally spend that much in a year at the science center, I can feel pretty good about spending that much considering how much more this package gets me versus normal pricing.

Here's a run down of savings for me in two visits with three kids and three adults in each visit:
OmniMax = $(6*$8)+(4*$6)+($3*2) = $78
Discovery Room = $3 * 12 = $36
MINDSTORMs = $3 * 12 = $36
Planetarium show = (6 * $5) + (6 * $4) = $54
TOTAL = $204, plus every visit hereafter

Granted, I admit I would never spluge that much at the Science Center and also didn't include parking savings, that's a lot of fun in just our first two visits for $85 (plug $8 for the two additional OmniMax tickets). They also have coupons near the Planetarium for 4-person buffet at Sweet Tomatoes for $20, which we'll be trying this weekend.

Particularly awesome is we can now take the kids' friends and get them into a great deal of extras for free, so even if the event is slightly boring for my own boys, they still get a kick out of treating their friends to the experience, and manage to still find new things.