There are several other voxel engines besides PolyVox. This page takes a brief look at some of them.

Voxlap

Website: http://www.advsys.net/ken/voxlap.htm
Description: Written by Ken Silverman (author of Duke Nukem 3D's Build engine), Voxlap was one of the earliest voxel engines. Even by todays standards it is one of the most feature complete as it allows large volumes, arbitrary destruction, integrated physics, and a voxel animation system. Sadly it is no longer developed, but the source code is available for anyone who is interested. It is also being used for the controversial though technically impressive Voxelstein3D.

Atomatage

Website: http://www.atomontage.com/
Description: One of the most promising commercial voxel based engines, Atomontage combines voxel and polygon geometry for both the rendering and the physics. It's graphically impressive and also allows real time modification of the voxel data. There is no demo available so it's hard to know how well it really works, but this one is definatly worth keeping an eye on. A dev blog is also available here.

Website: http://www.unlimiteddetailtechnology.com/
Description: This rendering system has generated a lot of debate in graphics circles, mostly due to the creators bold claims yet lack of specfic technical detail. Environments appear very though apparently static, and simple animation has been demonstrated. Their website presents some interesting images and videos, but again there is no demo available.

Website: http://artis.imag.fr/Membres/Cyril.Crassin/
Description: Graphically one of the most impressive voxel rendering engines, GigaVoxels is based on raycasting a sparse voxel octree on the GPU. There are a number of papers and presentations available online and a detailed article in GPU Pro. These cover both the basic rendering architecture and also special effects such as Shadows and depth of field.

Website: http://research.nvidia.com/publication/efficient-sparse-voxel-octrees
Description: A project by researchers at NVidia investigating a voxel rendering approach based on 'Sparse Voxel Octrees' and raycasting. Their work is particularly interesting because they have published a paper describing their research and have also made their source code available under the Apache license. Their solution is fully static and requires a long precomputation step, but they do also provide tools for creating voxel models from 3d meshes.

Website: http://open-ra.org
Description: An open-source implementation of the Red Alert engine using .NET/mono and OpenGL. Runs on Windows, Linux and OSX. It supports Tiberian Sun and Red Alert 2 Voxel graphics since playtest-20130629.