1. Vampire Diaries
Nina Dobrev who is playing the double role of Elena Gilbert and Katherine Pierce is dating Ian Somerhalder aka Damon Salvatore.
Paul Wesley and his wife Torrey DeVitto worked together in "Killer Movie" and in "The Vampire Diaries" as Stefan Salvatore and Meridith Fell, "Army Wives" as PFC Logan Atwater and Maggie Hall, though Paul was in Season 2 and 3 whereas Torrey DeVitto is a main character in season 7.
Emily Irene VanCamp who plays Emily Thorne in Revenge is dating Josh Bowman aka Daniel Grayson.
3. Twilight Saga.
Kristen Stewart aka Bella Swan and Robert Pattinson as Edward Cullen had been an on and off couple ever since they met in the set of Twilight.
4. One Tree Hill
Chad Michael Murray (Lucas Scott) dated Sophia Bush
(Brooke Davis) and married her after two years but they split in five months and before their divorce could be finalized Murray started dating Kenzie Dalton who played an extra cheerleeder in the serial. Now they are engaged.
Also Sophia started dating James Lafferty (Nathan Scott, Lucas' half brother in the serial.) Then she dated Austin Nicholas (Julian Baker) but they too broke up. Phew! And I thought the relationships in the serial were messed up.
Hilarie Burton who plays Peyton Sawyer married the show's assistant director Ian Prange but now they are divorced.
5. Snow White and the Huntsman
Kristen Stewart who played the role of Snow White had an affair with the director of the movie Rupert Sanders who was at that time married to Liberty Ross who played the role of Snow White's mother Queen Eleanor. Now they are divorced while Kristen is back together with Robert Pattinson.