Google Contributor, Kachingle, Webpass, Brave, Flattr, Blendle and I have probably overlooked a bunch. There's also a deadpool with Sprinkepenny, Contenture, Readability and -- again -- I've probably overlooked a few.

The core problem is preventing two classes of attack:

1. Visit multiplication by publishers, intended to over-represent them in the monthly scoring.

2. Money-laundering of stolen cards by using bots to sign up and visit an attacker-controlled site.

There is also the matter of preventing a publisher in the middle from tracking users by piggybacking on shared identifiers.

To my very great shame, I've spent too much of my time solving these problems rather than working on the business side of things.

Edit: at least it's another Australian :)