Trello by Fog Creek Software is a customizable planning and task management service that lets individuals and work groups get things done in a more organized and productive fashion. The Trello app for iPhone and iPad lets you receive notifications of added tasks, status changes, and more. Whether you're managing a group of contractors, working on projects yourself, or tracking sales, Trello is a great option.
The Trello service itself is web-based and can also be accessed from any web browser. For iPhone and iPad users, you can download the free Trello apps and access all your boards and cards as well as receive notifications when items get added or when the state changes.
The thing I've found so great about Trello is the customization levels it provides to work groups. You can change and format lists and boards however you'd like, add checklists inside a task card, and more. Inside an organization you can grant users permission to only certain boards if you want. It's an ideal option for small to medium size work groups that need a better handle on organizing, planning, and delegating. The iPhone and iPad companion apps make it even easier.
Trello also offers a business plan for organizations that gives you more options for customizations, the ability to import existing Google Apps users into Trello, better read/write access, and more. Plans are $25/month for an organization or $200 a year. If you don't want to go pro, you still have unlimited access to boards and cards, just not as many customizations.
If you use Trello or happen to give it a try, let us know what you think and what you're using it for in the comments!