Basecamp - from 37 Signals
37 Signals are a Chicago based company that make damn good web applications for small to medium business.
We use Basecamp to help manage our projects internally. It's clear and easy to use. It covers to-do lists, file sharing, tracking time, scheduling milestones in a project and your client contact.
There's a lot of software out there that will do the same thing, though we (like many others) think these try to cram in too many bells and whistles. We don't need to track the amount of ink spent in printing up layouts nor do we need a bar graph for my coffee consumption.
37 Signals also coined the idea of the Fortune 5,000,000, aiming their software at the small or 'side business' market.