Template Monster is one of the most popular template selling businesses on the Internet, they also have a killer affiliate program. Their program is extremely popular with webmasters and there are so many people promoting it that the field is quite crowded, however due to the features and rates they give you it makes this program a great choice for passive income creation.
The best part about the program is the commission, 20%. Also, to differentiate yourself Template Monster allows you to mark down your templates and have it come out of your commission. So you could sell Template Monster's templates for 10% off and receive 10% commission on it. Also, just by being a member, you can get 20% off all templates you buy from them.
As far as implementation tools go, I often say Amazon.com has the best implementation tools of any affiliate program I know, I would have to say that Template Monster is right up there though. In addition to standard banners and links they have very effective and easy to use tools for making an interactive datafeed-like website displaying their entire inventory, or just to display a handful of templates dynamically much like Amazon.com's "Recommends" links. If those options are still too hard for you though, they also provide ready made affiliate shops, basically you just upload those and you're good to go.
I made a simple Template Monster datafeed-type site a few years ago. With nothing but standard promotion methods, and probably most importantly a signature link in my signature at webmaster forums, I have managed to make around $300 a year off of it. This doesn't seem like much, but when you consider it was less than 4 hours of work to get the site going and that my only cost is the domain, which is cheap, that's a good deal of passive income.
However, the people who will have the most success promoting Template Monster will be those who run webmaster related sites.
The own downside is the payment options. Template Monster has somewhat of a shady past, they were kicked out of PayPal. They offer payment by direct deposit and wire transfers, but its hard, for me atleast, to trust them with my bank information. They also offer payment via MoneyBookers and check, which are more reasonable. I'm sure I'm just a little over cautious, they are a large company and it would behoove them to be as legit as possible. Still, its a minor issue and I fully recommend their program to everyone.