The Helix comes up more often than you may realise.

-On the cover of the Isaac Mendez-drawn comic book 9th Wonders!, where hiro cheers his teeportation to New York, "yatta!")
(Don't look back)

-On the map in Chandra Suresh's apartment
(Various episodes)

-On the sketch Peter draws in hospital
(Dont look back)

-In a crime swimming pool that Matt Parkman investigates.
(Don't look back)

-On the cover of Chandra Suresh's book about genitics and superhuman abilities: Activating Evolution.
(Various Episodes)(Picture shown below)

-On the side of claires Geometry textbook
(One Giant Leap)(show below)

-Drawn all over photo's and newspaper clippings on the amp in Sylars appartment.
(One Giant Leap)

-As necklace pendants worn by both The Haitian and Peter Petrelli.
(Various Episodes)

-On the hilt of Takezo Kensei sword (and on the toy replica Hiro plays with as a child)
(Various Episodes)(Picture shown Below)

-As a tattoo on Jessica's sholder(Though not her alter ego Niki's) and on Peter's arm in season 2
(Various Episodes)

-On various notes left on computers and desks relating to people with special abilities.
(Several Episodes)

-As Graffitti in a Utah bus stop where Micah waits.
(Homecoming)

-In Chandra Suresh's lizard tank
(Godsend)

-In a plate of pasta eaten at a italien restaurant, Where Claude swipes a cell phone(Mobile)
(Godsend)

-Throughout the offices of the primatech Paper company
(Various Episodes)

-Within the Petrelli law offices and Primatech Paper logos.
(Various Episodes)

-On the door of the Bennet's home, where Ted Sprague goes Nuclear.
(Company Man)

-On the floor of Chandra Suresh's appartment, made up of wooden remains of a smashed bookcase, following a fight between Sylar and Peter Petrelli.
(0.7%)

-On the banners of Takezo Kensei's army. (also seen at the Jittetsu sword store in New York)
(How to stop an Exploding Man, aeveral season 2 episodes)

-Scribbled on the photo's of founding members of the company who were targeted for death by Adam.
(Various Episodes)

-In several of Molly's drawings of her dreams.
(Four Months later)(shown at bottom)

-In Molly's eyes as she see's the 'nightmare man'
(The Kindness of strangers)

-Above the door in Montreal where Peter(and Caitlin) search for his destiny.
(The line)

-On the cover of 9th Wonders! Introducing "St. Joan"
(Fight or Flight)

-On Peter's arm after destroying the virus at the Primatech vault
(Powerless)

-On Nathan/Sylar's ring when he is on the phone to Matt Parkman
(Five Years gone)

-On the jewlerry of Hana and Guillame in the heroes online comic.

-As a winged surpent (from Hana Forklore) in the online comic

-In formations of rocks and insects throughout the graohic novel.

-In a crossword puzzle found on Hiro's blog

-Throughout the artwork in the Linderman archives of his Hotal's website


Let me know if i have missed any.....