37Signals have an ok product. What makes it truly successful is their incredible marketing machine. From highly popular blogs to podcasts, videocasts to books, super-optimised A/B tested websites to entire, hugely popular web development frameworks associated with them... they've got their marketing to a level that takes a decade of intelligence, opportunities and hard work to achieve.

"Basecamp is the top choice for entrepreneurs, freelancers, small businesses, and groups inside big organizations."

Total marketing. Says nothing about the product... but it connects if you're one of these types of people, making you feel like they must know something regardless of what the product even is.