[BRadmin]

37signals offers a 30-day free trial for paid plans, but clearly states: "If you cancel a paying plan within 30 days of signing up you won't be charged a thing. We'll send you an email 5 days before your first charge to remind you."

In addition they also offer FREE plans for their services - which require no CC at sign-up and allow you to upgrade at any time.

Edit: Any comments on the downvotes would be helpful as I was just trying to supply more detailed information on the actual process being discussed.