Expanding on their brick by brick, basics for free, pay for what you need approach, the guys at 37 Signals are now offering an Affiliate Program where users can earn free services:
The Basecamp Affiliate Program allows you to earn credits that are applied towards your Basecamp account. These credits reduce your subscriptions fees and allow you to earn free service. It’s your reward for helping us spread the word about Basecamp. EVERYONE who has a Basecamp account is eligible! When someone uses your affiliate code/link to sign up for a new account, you’ll receive a credit on your account after they’ve paid for their first month.
It’s not particularly new in itself, but for products like Basecamp and Backpack it makes a lot of sense. The basic features of these products — some of which are hived off as separate tools, like Writeboard and Ta-da List — are free but users who like the tools will quickly find they want the paid product. Now they can by corralling others into coughing up.