Me and my wife have finally found Listary to be the best option for simple shopping lists that sync. It's based on Simplenote's free service. We're so happy with it that I posted a short review as a test article on my revived blog:
http://www.buntbinaer.de/2011/06/05/listary/
I hope it won't remain the only article there for a long time, but unfortunately I've got tons of other things to do. But who am I telling this, it's the OmniFocus forum... ;)
Cheers,
buntbinär