iPhone: There's already a glut of to-do list apps available on iOS, but Listacular is a new entrant into the fold that brings a beautiful interface, Dropbox syncing, and collaborative list editing to the table, and it's definitely worth a look.
Once you open the app, just hit the plus sign on the top right to create a new list or new item. When creating an item, an extra row on top of the keyboard lets you make it a checkbox, bullet point, or plain text in a variety of sizes. You can also indent items to create nested lists, which is much-appreciated by this longtime Clear user.
Once you're done creating items, you can swipe them to the right to mark them as complete, swipe far to the right to delete them, or swipe to the left to add due dates and alarms. It's (very) reminiscent of Mailbox and Clear's gestures, but it works really well here.
One fresh new gesture is pull to sync. Pull your list down, and instead of refreshing, the app will commit your changes to a Dropbox text file. If you share this file with friends' Dropbox accounts, they'll be able to collaborate on the list through the app. For example, I'm using this with my girlfriend to create a shared grocery list, and so far it works really well.
Listacular is free, but a one-time $2.99 in-app purchase will let you create more than three lists at a time.