Presidents accused of using their power for personal gain include Richard Nixon and Warren G. Harding. Additionally, there are otherwise good presidents who are accused of stretching their presidential powers to the limit at certain points during their tenure.
Nixon created a list of his political enemies and used his governmental authority to harass them. He even went so far as to recruit a squad of secret operatives to spy on them. These activities culminated in the Watergate break-in, which eventually brought down Nixon's administration. Harding transferred oil assets so they could be sold off by his cabinet (The Teapot Dome Scandal) and granted plum political appointments to his friends and supporters.
Even respected presidents have been accused of abusing their power. Abraham Lincoln suspended habeas corpus during the Civil War. Franklin Delano Roosevelt interned thousands of Japanese-American citizens. President Truman illegally seized steel mills to break up a strike in 1952. Andrew Jackson defied the Supreme Court and had the Cherokee nation forced onto reservations in Oklahoma, now known as the Trail of Tears.