Scultpris is a free, cross-platform (Win+OSX) 3D modeling and texturing software, from Pixologic, famous for their ZBrush sculpturing top software. It allows you to work with e.g. Wacom tablets and can handle both 3D meshes and the texturing, although you can combine this with Photoshop and other 3D software.
Info on http://www.pixologic.com/sculptris/ (current version is Alpha 6, indicating it is not fully finished)
While the ZBrush software is seen as the logical "next" step, when you outgrow Sculptris, it is already quite complete:
|After some minutes dabbling with a small Wacom tablet|
|Real example from Sculptris website (artist: Barry Croucher)|
- dynamic tesselation (refinement of the mesh when sculpting) to support details in the regions which require it, without having to overwhelm the rest of the model with more smooth, flat surfaces.
- painterly-like interface, dark color scheme, icons, lots of rounded edges. If you use this only occasionally, this might be a disadvantage, though.
- several brushes and modeling aids (e.g. symmetry mode).
- custom brushes, based on patterns with alpha channels.
- masking support, to protect areas of your model.
- automatic UV creation, which ensures that the model has a well-structured set of texture coordinates, to assist the texturing on a usable texture map. This will also help with imports in other 3D software, e.g. for rendering or animation.
- support for texture projecting and bump-map painting.
This kind of software (Pixologic Sculptris and ZBrush, Autodesk MudBox) is mostly used in organic character modeling and design, but nothing prevents you from trying to find a use for it in architectural modeling and visualization. BodyPaint (as included with Maxon Cinema4D) is different, as it is a "paint-on-model" software, allowing you to work as if you where in Photoshop but on the texture map and live on the 3D model. ZBrush and the like will actually model and modify the geometry as well (the mesh).