I have one at the moment, and want to keep it that way. Omnifocus.
Most of my tasks come in via email from customers, so I have an Actionscript which will add any email message in a certain folder in Apple Mail and create a new task for it in Omnifocus. So my folder is called "Follow Up", then every 15 minutes anything in here will be a new action in Omnifocus. So all my tasks are in Omnifocus.
As I process email, if I need to create a task from it I move it to the Follow Up folder.
If I have a new task that does not come from someone emailing me, I either add it to OF myself (on Mac or iPhone) or more usually just email it to myself - I created a rule to put any email from myself to myself in my "Follow Up" folder.
So everything goes into OF directly or into the Follow Up folder which then goes into OF.