We've just made a new release of our PolyVox voxel terrain engine. You can read about PolyVox here and see a list of projects using it here.
Some key points:
- Written in C++ and available under the zlib license.
- Independent of graphics API or rendering engine.
- Can create both smooth and cubic-style terrain.
- Raycasting support for picking and ambient occlusion.
- Support for large volumes with compression and paging.
- Provides image processing operations (resampling, blurring, etc).
- Has an A* implementation for pathfinding through volume data.
Hope someone finds it useful :-)