Recs.
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Things provides features with handling for recurring to-dos, tags, notes and you can organize the to-dos into projects. There is a client for OS X and you can sync it with your iPhone / iPad using the Things Cloud.
Prices start at $9.99 and range up to $49.99.
SpecsUpdate
Pros
Pro Cloud sync
Users need to create an account for Things Cloud (the account can be created from any device). Once signed up, cloud sync will be active, and you’ll have to login on all of your devices to complete setup. From that point on, syncing happens in the background. It is invisible and always reliable.
Pro Project support
Things 2 has a projects feature that allows you to organize your tasks together. You can select a project you need to get done and Things helps you focus on each task to get it done. You can even group projects together to focus on the most important aspect of your life at the time.