What are the list of presidents that have been impeached?

Two US presidents have been impeached: Andrew Johnson and Bill Clinton.

Both were tried by the Senate (as is required in impeachment) and acquitted. Clinton was acquitted by a very wide margin - a 2/3 majority vote of the senate is required to convict, and on each charge, no more than half of the senate voted to convict him.

Johnson, on the other hand, was acquitted by only one vote.

Richard Nixon chose to resign rather than face impeachment. The House of Representatives was already holding hearings considering whether to draw up articles of impeachment against him, and then the Supreme Court ordered the release of several recorded conversations showing that he knew about the cover-up of the Watergate break-in, and tried to stop the FBI from investigating.

In the face of this, it became certain that he would be impeached, and, most likely, convicted and removed from office. He therefore resigned to avoid complete disgrace.

It is only speculation regarding former US president Richard Nixon.