Most donation centers won't take used carseats because they can't guarantee the safety or if it has been in an accident. I was given a carseat that was 8 years old. I waited until Babies R Us had their trade in event and got a 25% off coupon and bought DD#1 a convertible seat. They usually have a trade in event 2 times a year.
yeah, I'd do the recycling thing Josy mentioned. If you need the discount, the BRU event is usually held in February and August each year.
Otherwise, cut the straps and beat the crap out of it and throw it in the trash.