1. Abraham Lincoln. It turns out that in the 1950's, mystery cults devoted to Abe were spreading throughout China. The priest class of these cults was expected to emulate Lincoln in every detail, and breeding and genetic alteration programs were established as well by high-ranking members. Early emulations had mixed results, but today the various cults claim no less than forty-five individuals who perfectly emulate Abraham Lincoln in appearance, personality, memory and mindset.
2. Andrew Jackson. Shocked by the 1993 death of George Herbert Walker Bush at the hands of aliens, top Disney scientists sought to improve their Hall of Presidents. The fully holographic and interactive simulation of Andrew Jackson was not the best in quality - the best images were those in the secured back room of dead presidents still thought alive to the public (Carter, Reagan, Ford) - but when the Florentine necromancers went on a vacation to Disneyland and things got a little wild, Andrew Jackson's soul came to inhabit the hologram and the rest is history.
3. George Washington. This one's obvious. I mean, he's ageless and indestructible; of course he's still alive.
4. William Taft. He, of course, is hanging out in the Fat Guy Shangri-La with Elvis. The fountain of youth there actually contains naturally-occurring telomere-reconstructing bacteria. The restaurants are, of course, superb, if pricey.
5. Calvin Coolidge. Okay, this is technically a fluke. He really died like on the history books, but he had a secret vice of vacationing in time. Using Doctor Lectro's Patented No-Wax Flux Coil, Calvin happens to be vacationing in Hawaii from last summer until Christmas. Still, he qualifies as "alive today".
6. Zong Grenzber. First Alphan Grey President, elected in 2120, impeached in 2122 for violating of the forty-third amendment. In Hawaii with Calvin after being an ex-president. Time travel vacation, as well.
7. Millard Fillmore. His brain was transplanted into a young tortoise at his death. He's on his sixth tortoise now, and getting cranky. His re-animators recently agreed and signed a contract which states that his next body will be a giant robot tortoise with "acid missiles and turbo".
8. John Adams, aka John Quincey Adams, aka Ansel Adams, aka Adam Corrola. I don't know if this counts as one ex-president or two. Still alive thanks to rare Taoist alchemy, John has been switching identities since he claimed to be his own son. His nature photography career was spectacular, and now he is one of the hosts of "The Man Show".