Zenbe has become my de-facto to-do list - beating Google's and any sort of Apple solution - for two reasons: ease of use and cross-platform compatibility. I can make grocery lists on the computer and check them off on the iPhone at the supermarket.
This is what it looks like on the iPad. Sure you can just hit up the Zenbe site on iPad Safari, but this is more optimised. And, if you already have the iPhone version, you get this for free. It's $4 for both otherwise. [iTunes]