Tomiko Asahina (From the New World); via lengthening her telomeres
Master Son (Gintama)
Winifred, Mary and Sarah Sanderson (Hocus Pocus); via life potion
Her Imperious Condescension (Homestuck)
Madara Uchiha (Naruto)
The Childlike Empress (The Neverending Story); via new names
Ziralt (The New Gate)
Various Enlightened (Reikenzen); via Levels/Ranks Of Understanding
Machinians (Space Dandy)
Half-Elves (Tales of Symphonia); via Cruxis Crystals
Redmond, Blutarch and Gray Mann (Team Fortress 2) via Life-Extension Machine
The Administrator (Team Fortress 2) Via Life-Extension Machine
Lachesis (Valkyrie Crusade)
Blood Orchids (Anacondas: The Hunt for the Blood Orchid)
Heart of Atlantis (Atlantis: The Lost Empire)
Lazarus Pits (DC Comics)
The Glory (Doctor Who); to its controller
Paradise Herb (Dragon Ball Super)
Elixir of Life (Harry Potter)
Cloak of the Starmaster (Homecoming Saga)
The Fountain of Youth (Pirates of the Caribbean: On Stranger Tides)
Elixir of Life (SCP Foundation); formerly
Life-Extension Machine (Team Fortress 2)
By continuously bathing in the Lazarus Pits, Ra's al Ghul (DC Comics) has extended his life for over 600 years.
Paradise Herbs (Dragon Ball Super) extend the life of whoever eats them, and are the source of Master Roshi's longevity.
Toki (Fist of the North Star) was able to use his knowledge of pressure points to extend the mortally wounded Rei's life by a few days.
By continuously lengthening her telomeres, Tomiko Asahina (From the New World) has extended her life for over 200 years.
Master Son (Gintama) using Qi Gong to manipulate his lifespan, having lived in the battlefield for more than two centuries.
The Elixir of Life (Harry Potter) will extend the drinker's lifespan, so long as they continue to consume it, but it won't cease their body from getting older, so they will continue to age, and will eventually be completely dependent on the Elixir to live.
Madara Uchiha (Naruto) extended his life past his natural lifespan by attaching himself to the Demonic Statue of the Outer Path, but his body continues to age.
Lachesis (Valkyrie Crusade) can extend the years of life of anyone she wants as much as she wants.
Heart of Atlantis (Atlantis: The Lost Empire) provides phenomenal longevity, with the ability to extend an individual's lifespan by almost 500 times.
The Elixir of Life (SCP Foundation) is a liquid SCP-1440 won from the Small Death that could extend even the sickest person's life with a single drop until SCP-1440 used all of it to save various people from dying.
Perrinia immortalis/"Blood Orchid" (Anacondas: The Hunt for the Blood Orchid), contains a unique, naturally generated chemical that extends an organism's telomeres, enabling them to live longer...
... the main reason why the anacondas are unnaturally larger then they should be because they keep growing till they die.