From the HTTP specs regarding the referrer headerfield (emphasis mine):
The Referer[sic] request-header field allows the client to specify, for the server's benefit, the address (URI) of the resource from which the Request-URI was obtained [...] The Referer field MUST NOT be sent if the Request-URI was obtained from a source that does not have its own URI, such as input from the user keyboard.
The above-mentioned sites all have spiders crawling websites, and in violation of the specs they all pass an URL to their own website in the referer-field although none of them has obtained the URI to my site from the URI they are passing (do you see a link to me at http://organica.us/r? Didn't think so).
My initial guess was that the misuse is a failed attempt to explain what their spider is and where it is coming from (conceptually the spider is coming from ie http://www.popdex.com/ but it sure didn't obtain my URI there). However another field exists that can rightfully be used for this purpose; The user-agent field - put links to your website there.
It turns out that at least Popdex and Organica already utilize the useragent field for this, so I am left to wonder why they're messing with the referer field. Ignorance comes to mind, luckily that can be remedied.
It annoys me that people opt to pollute my referrerlogs with fake data. I am especially surprised by the 3 above-mentioned sites doing it, seeing how they all rely heavily on the blogging community, that generally doesn't look mildy on misuses like this. News aggregators have been taught to behave, hopefully the spiders are next.