Since I have a mac, an iPhone and an iPad, it is a very nice thing to be able to add and subtract data on any of the three devices and have it update across all three.
So far I have used myNetDiary, which has an iPad and a website version that are very good at syncing with each other. I also use and like GroceryIQ, which has both iPhone and iPad versions, making it easy to do my changes on my bigger iPad, but carry my more portable phone to do my actual marketing.
So what other apps have multi-device versions that sync? I'm interested in anything.