Minecraft Forge: Any mod like Multiverse?

You would probably be looking for something like Mystcraft. There isn't necessarily a free transport between the worlds, but you can create portals with crystals that you find in some of the Mystcraft "Ages". You can also create what's called linking books and use those to teleport around.

I know of one: Mystcraft

It isn't "free" transit, especially not in the more recent versions, but once you have everything together traveling to other dimensions is easy and otherwise cost free.

You must first create the books. You must then find the appropriate pages for the books, crafting most of them out of paper and squid in on a special work bench. Then create a book stand and put the book on it. You can now use the book to travel to another dimension.

Note that many (read most) of the early dimensions are unstable. You will have to find additional pages (villages have them, so do dungeons) to be able to make a better book, which will be more stable. Eventually you will find enough pages that you can build a completely stable book.

Important note and survival tip: Before entering a new dimensions *ALWAYS* craft a linking book that is connect to the overworld. Without one you can easily be stuck in the other dimension, as many don't have a natural way back out.