Occasionally we send out referrals for other software from fellow Indie Mac developers in our monthly mailings. As a way of helping our users/friends to find out about cool software, and also as a favor for our developer friends, we will include a blurb about the other developer's product(s) in our mailing. Often we'll arrange with the developer for a coupon code we can mention, so that our readers get a special deal by buying the application using our referral.
Sometimes this is a one-way thing, where we just mention the application because we like it so much, for the good karma of doing so. Other times, we have made agreements with the other developer that they will put in a reciprocal mention of our product in their mailing, so that we scratch each other's back.
Of course it's quite possible to set up a commission kind of deal for this kind of thing, though we have only done this once or twice. As long as it's still a genuine recommendation, I don't have any ethical problems about doing that; we all have to make a living and doing so is a way to bring in another stream of income.