Voxels In Time is a mod where you can travel across versions via time portals. You can also discover new stuff, although that aspect of the mod is currently not fully there yet. The mod is currently unfinished, but you can still try it as I work on it.
Download the latest version of the mod at my site.
VIT "Classic". The old stable build. Very basic compared to latest experimental, but with less bugs and supports Multiplayer.
Latest developmental version. No Multiplayer support, but adds many new features, options, dimensions, and many more things. WIP.