Due to the Resonance Cascade and the Perpetual Testing Initiative, entrance to other worlds in other dimensions is possible. Here is a list of worlds encountered in the official series or mods.
Good old mother Earth, which got reduced into nothing more than a giant, nearly dry wasteland after the Combine invasion. So far, it's the battlefield between the Combine oppressors and the Resistance.
The Moon is more of a satellite, but because of it's appearance in Portal 2, and the lack of other satellites in the Half-Life franchise and mods, it is included.
Described as The Borderworld by the G-Man, where the Nihilanth, it's vortigaunt slaves, and other alien races reside. A large amount of species were teleported during the Resonance Cascade, and this world fell under G-Man's control at the end of Half Life.
A strange world acting like a purgatory of souls, where deformed scientists test sadistic trials upon the poor humans that arrive there. It is often black and white, but there is also grey and green wastelands, where some people seems to live, humans and Vortigaunts alike.
Random Quest's WorldEdit
A world where the Vortigaunts seems to be powerful tyrants, and humans are their soldiers or slaves. So far, we've seen a Middle Age Castle, a ruined Coliseum, an Arena in the wastelands, and random interlopers.
G String's WorldEdit
Earth, if totalitarian regimes have conquered the whole world, and pollution, technology, and wars ravaged everything.
Get a Life's WorldEdit
Earth, under a giant zombie pandemic.
The Multiverse Edit
An infinite amount of Earths with various differences. All thanks to the Perpetual Testing Initiative in Portal 2 and the Resonance Cascade in Half-Life. Also home to the Money Planet.