With Valve's breakout video games, Portal and Portal 2, attracting so much attention as of late, it's easy to forget that portals are something of a time-honored tradition in science fiction and fantasy. So, in the spirit of inclusiveness, we've reached outside the orange box to bring you this collection of concept art, fan art, and stills from some of our favorite windows in time and space.

Remember, we're talking portals here – seamless passageways between time, space, and other worlds. Ofelia's chalk door in Pan's Labyrinth? Yes. Jeff Goldblum's telepod in The Fly? No. Now discuss!

Top image is "Steampunk Portal" by Risachantag at deviantART

Doctor Who - "The Girl in the Fireplace"
The Doctor, Rose, and Mickey land on a derelict spacecraft in the distant future. Throughout the ship, the Doctor and his companions find several "time windows" that allow them direct access to 18th century France. One of the only two-way windows is the fireplace, through which the Doctor and Mme. Pompadour can clearly see one another.

Doctor Who - "The Girl in the Fireplace"

Thor - Bifröst, also known as Rainbow Bridge
Bifröst is the only permanent portal connecting Asgard to Midgard
Via Marvel

Thor - Bifröst, also known as Rainbow Bridge
Concept art by Marvel for Thor's version of Bifröst, reimagined as a wormhole.

Harry Potter - The Vanishing Cabinet Via

Neil Gaiman's "Coraline" - The door to the Other World

Teenage Mutant Ninja Turtles - The Technodrome's Trans-Dimensional Portal
Housed in the Technodrome, Krang and The Shredder use the Trans-Dimensional portal primarily for traveling between Dimension X and Earth.

Transformers - Space Bridge
The space bridge is used predominantly by Decepticons as a portal between Cybertron and Earth.

Transformers - Warpgate
Warpgates, like the Decepticons' space bridge technology, generate spatial portals that allow Autobots to travel vast interstellar distances in the blink of an eye.

Monsters Inc. - Closet Door Portals
The specially-activated doors in Monsters Inc. become portals between the monster world and the human world.
Via CGSociety

Alice in Wonderland - The Rabbit Hole
"Steampunk Alice BG" by Karla Chan at deviantART

Star Trek:TOS - The Guardian of Forever
The Guardian of Forever is discovered by the crew of the USS Enterprise in 2267 on an ancient planet where all the timelines throughout the Milky Way converge. The Guardian serves as a gateway to the time vortex, making it a portal to other times and dimensions (in the above image, for example, the crew observes the American Revolution through The Guardian).

The Chronicles of Narnia - The Wardrobe

Beetlejuice - Chalk-drawn portal to the underworld

Stargate (SG1/Atlantis/Universe) - Stargates
You didn't think we'd forgotten stargates, did you? Stargates, like many other famous portals, work by generating artificial wormholes that allow one-way matter transmission and two-way radio communication between corresponding gates.