Fisheaters has much good information. However as you have suspected they are Sede Vacantists. Sede Vecante or the Chair is Vacant is authentically the period between the death (or in rare instances resignation) of one Pope and the election of the next. There are several Sede groups around, Fish Eaters is one who do not believe there has been a Pope since the death of Pope Pius XII.
Each group has it’s own conspiracy theory, some of the groups have even elected there own Pope. Among these anti-Popes are a former Auto Mechanic in France, a man who was expelled from a couple of seminaries and lives on his parents farm wearing his white cassock in Nebraska, (Fr. Lucian Pulvermacher, OFM Cap) who lived in Montana and decided that since there was no Pope, and no Cardinals that the laypeople who supported him could elect him Pope. The first “conclave” they had elected a teenage boy, he nullified it and held a second “conclave” via telephone and fax, then announced that he was elected as Pope. There is a fellow in Australia who calls himself the Little Pebble, who is waiting for a sign to announce his Papacy.
While there have been times when more than one man claimed to be Pope in the history of the Church, it was sorted out, and the one man who was in fact Pope was able to reign. Unlike today when there is one legitimate Pope, (Benedict XVI) and dozens of false or anti-Popes claiming the office.
Jon our webmaster, and moderator makes sure that authentically Catholic discussions take place here. There are a few other sites that are authentically Catholic the one run by Karl Keating of Catholic Answers and Fr. Z at What does the prayer really say, are also good and faithful to the Magesterium. What I like about this board, even though it is not as active as others is it’s small size. Sometimes the other boards can get so busy that it’s hard to really follow.