I'm currently seeing pleasing results promoting Channel 4's 4oD via TradeDoubler. 4oD is a free TV catch up service, allowing users to watch programmes from the Channel 4 archives. It's very useful for watching shows you missed, it's free and best of all, Channel 4 will pay you a pound for each user that downloads it. Seriously, does affiliate marketing get any easier than that: you have a great product, available for free and it's something users will want when they know about it or might be looking for anyway.

Similarly, Firefox via Adsense Referrals. Firefox is a great app. Chances are that you'd recommend it for free anyway, but Mozilla/Google are willing to give you up to a dollar for each person that downloads it. Some people have to go door to door trying to flog things to people who don't really want them. All you have to do with Firefox is tell people how cool it is and they can download it for free. And if you're going to link to Firefox, why not "upsell" them to the Google Pack, which contains Firefox and other good stuff like Picassa, Google Earth etc and has a payout of up to $2.

A few years ago you could get 50p for promoting Yahoo! Messenger on TradeDoubler. Again, a useful tool that people are actively looking for everyday - all you had to do was guide them in the right direction.

Finding programs which pay you to promote genuinely good tools that are available for free is a real no brainer. Sometimes there's "free" things you can promote but you wouldn't really want to promote them other than for the commision, but when it's a genuinely great resource, like 4oD, or Firefox, or Google Pack, it really is a wonderful thing to get paid to promote it. Have you got any examples of really useful free products/downloads that have a payout attached?