I have tried many todo applications over the years and never found one that quite met my needs, so I decided to launch my own. My personal workflow is all based around priority, specifically I want to focus on what I want to do today and what I will do tomorrow, with everything else going into a backlog that I will deal with later.
In addition to having a narrow focus on today and tomorrow, I also wanted a system that would attempt to suggest items for me to tackle next, based on the priority of the item set by me during creation, and the amount of neglect the item had received to date. The amount of neglect is an important factor, because you do not want items in your backlog that effectively never get looked at and basically starve.
Finally, rather than dumping everything into today or tomorrow and overwhelming me, I wanted something that would encourage the user to stick with five todo items per day, which I believe based on experience is an achievable number, therefore Five.Today was born.
While still in alpha and not feature-complete, the application is useful enough at this stage that I use it every day, and I will continue to improve it. I hope that other people find it useful too!