I think there are ethical issues here that other people have not brought up, even after the refereeing job is done.

By letting the author know about your (very positive?) referee report, they may feel more likely to "repay the favor" in the future. This seems potentially problematic to me (although in many cases it won't be). Obviously, if you recommended rejection :-( or gave an isufficiently glowing report, this seems less problematic from an ethical perspective...

That said, I have revealed that I'm the referee of various papers in the past. Although usually because something came up in conversation and it seemed natural to mention this (ie, "I know what you are talking about, I refereed this paper of yours a while back and had a conversation with you about this topic through the editor"). To me this could still be problematic although perhaps still can be good/ok.

For example, in one case I revealed I had been the referee of a paper to the author after it had been accepted but before it had been published because I wanted to exhort the author to put the corrected version of the paper on the arXiv (to replace the existing version with some misleading mistakes).

Finally, in another case I revealed my identity to the author after getting permission from the editor during the referee process. This was a case where the editors were not looking so much for my opinion of the paper in terms of quality (it was a birthday conference proceedings), but wanted a detailed report on correctness. I asked the editors if it was ok to contact the author directly to speed up the process.