Life really is messy. I do my best to keep on top of things, tasks, notes, projects, contacts, etc and for that I have a project management system in place and on the whole it works well.
However, sometimes you still end up writing a quick task on a post-it note or back of an envelope and spend ages trying to find or remember what it was. This is where Clear by Realmac Software really works well. It offers a VERY easy to use, simple system that user gestures for interacting with your tasks. You simply pull down to create tasks, rearrange tasks by priority, pinch to see top level task lists and swipe when a task is done - giving you ultimate satisfaction when you complete something. Think tick boxes but with cool-iOS-slick-gesture interactivity.
Trust me, it is very addictive and getting through your tasks feels great!