For those that have heard about Basecamp, but never quite sure if it's right for them - the best way to find out is to use it. 37signals have always offered a Free Plan, although increasingly they've made it harder to find on the Plans & Pricing page.
Admittedly it's very limited, 1 project, no file sharing, which is why I always recommend people start with the Basic Plan - 15 projects, 5GB of file storage - and only $24 p/month. Now it appears 37 signals are keen to bump users up to the more-expensive Plus and Premium plans, so this too has been relegated to the bottom of the page. But not lost, so check it out if you want to get a low-cost taste for how Basecamp can rock your PM world..