Parallel Worlds!

"A parallel world is often a hiding place for unpleasant things, and often the protagonist is forced to confront effects of this other world leaking into his own." - Wiki pedia author.

Ok like you when I hear parallel world I groan! But the can be fun and interesting too!

Parallel World Technique
This technique is used to spice up a game and add a bit of wonder and open the PC's eyes to alternate worlds just beyond their perceptions. The main reason to use this is to take and existing dungeon or region and use the old maps and info but change it so it is new and different. The PC's are in a well known area and they are transported to an alternate world. This can be by magical accidents like to magic items striking each other. A tear in the fabric of the veil of the universe. A magical doorway or mirror. Or just taking the wrong fork in a path. The result is the same. The PC's are in an alternate world where things are similar to their world but there are a few differences. The reason this is a useful tool is that it allows the DM to break the monotony of the setting. Also the PC's get a change of venue. With delicate handing this can be a useful tool to both surprise and challenge player. This technique is best used sparingly and with caution. Nothing ruins a game more then inconsistency.

Rules
1. It is best used in an existing game where the agreed upon nature of the setting is established.
2. Do not over use or it will irate the players.
3. Always provide an exit back to the normal world and make it attainable to the PC's.
4. Never let the PC's over use the means to travel between worlds. Limit it. Or the PC's will be hopping back and forth all the time. This will cause the DM lots of extra work.
5. Make the parallel world different but not so alien that the players are annoyed.
6. Be consistency. Things should be just about the same in the parallel world each time the PC's visit. Normal everyday things change but not the main flavor of the parallel world.
7. Have a good parallel hook. Make the parallel world interestingly different.
8. Make the experience rewarding and fun for the players.

Examples of Parallel Worlds.
A world where there are only talking animals and no humanoids.
A ruined dark world filled with undead.
A fey world filled with magic both wild and beguiling.
A frozen wasteland filled with rugged survivors and Arctic version of common monsters.
An very similar world where evil is good and good is evil.