iOS/Web: Cheddar is a very simple to-do list manager for your iPhone, iPad, iPod Touch and the web. It syncs your to-dos all over and understands hash tags and Markdown.
Adding lists and items is simple; you just click the plus, create a list, tap the list, and do the same for each item. You can type it in Markdown or plain English. You can add a hash tag to any item by just typing # and whatever tag you want. When you check off completed tasks, they'll still show up on your list, but you can remove them easily by tapping edit and choosing to archive them. Alternatively, you can delete them from the list for good. Everything you do on your mobile will be available on the Cheddar web app (and vice-versa). It's great if you just want a simple to-do list that follows you to your many devices.
The one primary downside of Cheddar is that you can only have two lists. You can add as many items as you want to those two lists, but you're still stuck at two if you don't subscribe to their premium service. A paid account will only cost you $US20 per year, which isn't that bad, but it may be a bit steep for some when there are more feature-rich task management apps like Wunderlist. That said, if your primary goals are synchronisation and simplicity, or you really want a Markdown-capable to-do app, Cheddar is a good choice.
Cheddar (free, $US20/year for premium)