I'm involved with developing a modern voxel library called PolyVox which provides volume storage (including paging), surface extraction as well as supplementary features like ray casting and ambient occlusion calculation. It's not a game engine though but provides all the voxel stuff you need to plug into anything else. It's fully open source and there's a good developer community for it. On the forums people are always willing to answer general questions about voxel rendering etc.
