PPI model is very lucrative and first PPI affiliate I noticed was in 2003. Sometimes are these programs annoying for surfers but you can be surprised how much they can make for you. You are offering a small program which is automatically installed to visitors computer. For this installation you can get from $0.03 to $2.00 conversion ratio is from 1:10 to 1:40 so with 1000 visitors you will make up to $200. It is better than some impression programs because it converts every traffic. There is more and more of these Pay Per Install networks which offer bundle software for adult or non-adult websites. There are also PPI programs offering content locker, there you place simple script to website where you have valuable content and in order to get this content user must install some free software. Other option to make money with pay per install network is to create simple software and bundle it with PPI network SW exe file which would be installed together with your simple SW, than all installs are monetized in case user installs everything.