Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.
Four note boards - A simply stupid text tool to help think multidimensionally
Asana Boards - A simple, visual way to track your team’s work
Walling for Windows - Visual walls for your ideas and projects. Now on Windows.
Airtable - Airtable works like a spreadsheet but gives you the power of a database to organize anything. Sign up for free.
Slidepad - iPad style multitasking on your Mac