THE SILVER KEY
The Silver Key is the manifestation of Bob's power to serve as a bridge between worlds. It sends whoever uses it to a place where all their wishes have come true or into the cold, hungry, tentacled embrace of the ancient beings who inhabit the space between worlds. After it's been used, it disappears.
Bob has offered it to many people over thousands of years, but in the series, we've only met 3 of them: Professor Ingrid Tussmann, Chris, and Penny. In Mummy Issues, Prof Tussmann was looking for a place to hide from a murderous revived Egyptian mummy, but instead ended up being dragged into an inter-dimensional portal by hungry monsters. Chris wanted to go to a world where he was a famous actor, and instead ended up a complete unknown, even to his own family, in Penny's world. They met at group therapy in Penny and the Silver Key Part 1 and he told her his story, only a few days before Penny herself was offered the Key by Bob.
INSPIRATION
Obviously Lovecraft, though this Key is simpler - it just detects its user's greatest wishes and opens an interdimensional door to wherever matches that all on its own. The transference is also physical rather than just mental, with both Chris and Penny physically travelling to their destinations. And lastly but not the least, the Key isn't an independent object, but rather the temporary manifestation of Bob's power to walk between worlds.
THE NECRONOMICON
The dreaded Necronomicon is a book of eldritch magic that can power rituals. It's currently in the possession of a mysterious Master and his group of cultists, deep underneath Silicon Valley, where it's slowly infecting human minds with its particular brand of madness - fanaticism. It made it's debut in Geek Addiction where it was shown to be able to interfere with Bob's powers. The cultists refer to Alhazred as the Necronomicon's priest, and also link it to Yuggoth and Azathoth.
INSPIRATION
Lovecraft, again.
THE PNAKOTIC MANUSCRIPTS
They're seven scrolls containing secret ancient knowledge from the long-gone kingdom of Lomar. In Mummy Issues, H P Lovecraft tells Tim that Bob reached out through his dreams and took him to frozen Lomar where he showed him the Pnakotic Manuscripts. In The Heavenly Journey of Brother Lucas, a XIII Venetian monk is given one of the scrolls in Constantinople by a Yemenite named Abdul, who tells him it contains a ritual that makes it possible to reach Heaven. Instead, Brother Lucas ends up in the Dreamlands after spending days trying to decipher the scroll.
INSPIRATION
And yet more Lovecraft, though his Pnakotic Manuscripts were more numerous, and didn't seem to be themselves magic.
SELENE'S MIRRORS
Magical mirrors capable of reflecting people's inner nature, hidden fears, and secret joys. In Tim's Winter Adventure, Selene tells Tim that her mirrors show only the truth of what's in front of them. However, the biggest and most powerful one not only can look into other places, but also allows someone to travel through it.
INSPIRATION
The Devil's mirror from Andersen's The Snow Queen.