Nice discussion. I personally use http://tipit.to because I can actually get money out of that service, making it much more interesting as an alternative to an advertisement revenue stream. Tipjoy is not free and doesn't allow you to get any money out of the service, which make it a harder sell for me.

Secondly I personally think a tip service should definitely be a very social enterprise, and I have the feel that Tipit is that a bit more, with their "pledging of tips", etc. I think you'd just have to go and see for yourself.