Priority Matrix is an organisation app that I use every day.
It uses a classic four-square approach to prioritisation. The standard template lets you create four categories for your tasks: critical and immediate; critical but not immediate; not critical but immediate and not critical, not immediate. Once you assign a category for your tasks, a range of reminder settings are created for each one.
The basic version of the app is currently free to run on a single mobile device.
The developers have created Windows and Mac versions, so I now have the software running on my two windows PCs (home and work) as well as my Mac Book Air, keeping all fully synchronised. For this I pay an individual user licence fee of $10 a month.
There is a team version which I have yet to try, which allows up to 10 people to use the service for $79 dollars a month.