I hate the professional fundraiser company stuff -- so much of it gets skimmed off by the company, they sell overpriced junk, and the pressure on the kids is ridiculous.
Some great fundraisers our school has done, or that I've heard of:
Barnes and Noble bookstores will arrange to have a fundraiser night for your school -- a percentage of every sale goes to your school (you have to bring in the flier and request it), it's real books you'd buy anyway, and they will host a "read-aloud" with teachers from the school in their children's department that night. I loved this!
Our school had a Halloween pumpkin sale.
Carnivals are great -- ours includes a silent auction, but honestly I think that is a lot of trouble for not that much return. One thing at our carnival that has been a hit is that the kids in the afterschool program make craft projects in the weeks beforehand, then they're sold at a booth. Of course, everyone has to buy the adorable leaf-shaped soap dish or beaded pin or laminated bookmark their kid made, because AWWWW! They make a killing, and the kids are really proud of their stuff.