If your school has a uniform a second hand uniform shop is a good idea - split the takings between donor and school. Open the "shop" maybe twice a term eg in the school hall. Needs quite a bit of admin input but a great service and money maker.
My last school did a Friday cake stall - each class or year group on a rota to privide cakes each week, manned by Mums of that class/year. Flat fee of 20p per cake, place stall on way out of school for guaranteed sales to exiting children!
Stalls at sports day selling Pimm's and sweets / crisps etc bought for nothing at cash and carry (in fact make sure your school has a cash and carry card for everything they are great!!)
Quiz nights, discos, wine tastings, coffee mornings, curry night, scottish celidh all done over the years. A lot depends on the spending capacity of your parent cohort - you don't want to make them feel they are putting their hand in their pocket constantly.
Another thing I have found is that parents want to know where their money has been spent so if you can say it's for something specific eg electronic white voard, minibus or whatever then people are more willing to contribute in my experience.