Realm Crystal (Lego Ninjago: Masters of Spinjitzu)
Lor Starcutter (Kirby)
Gwen Tennyson (Ben 10)
Catbug (Bravest Warriors) midway through jumping into the See-Through Zone.
Wulf (Danny Phantom) can use his claws to rip open portals between Earth and the Ghost Zone.
Zandora's Box (Dungeons & Dragons: The Animation) can lead to any possible and impossible dimension, depending on where its opened.
Breach (Generator Rex) using her extra arms to open dimensional portals.
Grim (The Grim Adventures of Billy and Mandy) can use his scythe to slash open dimensional portals.
The Dimensional Scissors (Star vs. the Forces of Evil) are magical scissors that can cut open portals to other dimensions.
Hekapoo (Star vs. the Forces of Evil) the creator of the dimensional scissors.
Uncle Grandpa (Uncle Grandpa), among many other amazing abilities, can travel through various dimensions and planes of existence, and can even travel into other cartoon-universes as seen in the "Steven Universe" crossover.
The Magical Watches (Winx Club) can open portals to the World of Dreams/Neverland.