Above all other forms of making money online, there is the beast that is cookie stuffing. The biggest advantage is that not many have heard of it, so few are doing it overall. These few make hundreds of thousands, but also put themselves at risk of being sued (As Shawn of DP found out in '06 )
I won't go into details on the methods (you'll need to PM me for that), however basically you plant an affiliate cookie into a user's browser without them knowing. This means that if they join the said product within a set amount of time (usually 30 days), you are counted as the reference, and get paid.
On to the fun questions, do you do it yourself, and what do you think of the idea?
Note: It is not illegal, simply unethical.