A conspiracy is to be contrasted with a cabal. The two are similar but have quite different connotations; in contrast to a cabal, a conspiracy usually looks to overthrow a fixed power instead of usurping it from within.
The Catiline conspiracy, thwarted by Cicero as consul.
Two other famous conspiracies were the murders of David Rizzio and Lord Darnley, (Henry Stuart, 1st Duke of Albany), respectively Mary, Queen of Scots private secretary and her husband.
It is also generally considered that the infamous "gunpowder treason" attempted by Guy Fawkes was a conspiracy, although a thwarted one.