A list can be found here: http://www.scamdetector.info/ptc-scam-list.html
Forum on the subject can be found here: http://www.talkptc.com/index.php?action=forum
and here: http://www.gptboycott.com/forum/
The investigation list: http://www.ptc-investigation.com/pages/master.aspx
but check all those mentioned out against WOT as a guidance to sort out many a bad apple;
or do a check up here: http://www.dnsbltools.com/gtp-tech.com/
or here at http://hosts-file.net/?s=
polonus