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Good point bartek, enhancement. That's what I've also heard it referred to as.
I believe the current consensus is that banners are done for, but inline text affiliate programs can be workable for revenue to some degree, if done right and in context. You'll find good information and ideas in the Website Advertising Forum [webmasterworld.com]
austtr, I've heard mention of postcards on sites in conjunction with viral marketing techniques, a whole study in itself.
1) Make money from the sending of the cards with a tell-a-friend affiliate program. A couple of them listed here [webmasterworld.com].
2) Get signups for ezines/newsletters inline with the sending of the card. (Already setup with superTAF)
3) Put text ads in the card emails.
Depending if you run ezines or not, #2 can be very productive.