Monica Samille Lewinsky snitched on former President Bill Clinton when she testified before a grand jury that they had had a sexual relationship. That normally would not be snitching since it is not illegal for the President of the United States to have a relationship with an intern, but Bill Clinton had previously made sworn statements that he had not had a sexual relationship with Monica. Bill Clinton was later impeached by the House of Representatives.
Any normal person would have been found guilty of perjury based on Monica Lewinsky's testimony. The fact that she told all to the grand jury and that information was later used to impeach Bill Clinton is enough to qualify her has a snitch. Clinton was lucky not to get into more trouble over it. He could have been charged criminally later on.
The biggest snitch in all this was Linda Tripp, but she passed away, so Monica is now the subject of this profile. Tripp was the one that secretly recorded Monica admitting to the affair. She then gave those tapes to the special prosecutor and the rest is history.