Yes, very very many. This is a little-researched topic, sadly, but new studies into conspiracy theories as a historical phenomenon, like V.E. Pagán (2004), 'Conspiracy Narratives in Roman History' and J. Roisman (2006), 'The Rhetoric of Conspiracy in Ancient Athens have done a lot of research into the ways in which unlikely theories of collusion and corruption by actors hostile and manipulative of the public are to blame for all that's wrong in the world.
The word 'conspiracy' comes from Latin, from the marvellously sinister 'to breathe together'. In Greek, many words seem to have come to be employed as ‘conspiracy verbs’, with connotations of conspiratorial intrigue, such as epibouleuo (‘to plot, to design against’), paraskeuazomai (‘to prepare, devise, procure) among others. Common recourse to accusations of conspiracy are in the courts. For example, when defending a client, 'my friend was not being a criminal, he was actually investigating a deadly conspiracy against the democracy, involving ... the prosecution!'.
One topic which quickly began being discussed in terms of conspiracies in democratic Athens concerned the 'mutilation of the herms' (which I have discussed before). Who mutilated the herms? Nobody really knows, but Greek historians draw up theories concerning plots and entrapments to get their enemies executed or exiled.
Sources and further reading:
V.E. Pagán (2004), 'Conspiracy Narratives in Roman History By ' (see the Google books preview).
J. Roisman (2006), 'The Rhetoric of Conspiracy in Ancient Athens (first chapter available for free with this link).
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