There are not many things which impress us here at Site Sumo, however we almost fell off our chairs when we came across Trello!
This little beauty a MUST for any business which wants to keep track of ANYTHING.
Basically is breaks down to boards, lists and cards.

Board: A group of lists which you can use as clients / projects / locations / departments. We use the boards to break up each client for our Master Sumo service.

Lists: A list hold a group of cards and in a sub-section of a board. We use cards at Site Sumo for action steps that are assigned to each department.

Cards: These are little powerful buggers! You put a task on a card, however the power comes when you "flip" the card and you can add comments, checklists, labels, assign to users.. and lots more.

Highly recommended and we wonder at Site Sumo how we ever got by without our Trello!

PS: Its missing one critical feature. The ability to email tasks to Trello. So the team at Site Sumo are developing a neat little add-on called Emaillo that will enable you to email taks to Trello lists! (Will keep you posted on lauch date)