3D Modeling Tools
This is simply a quick list of tools I use in my 3D modeling/animation workflow.
3D Model Viewer
Open3Mod is my model viewer of choice, but there really isn't a perfect solution as far as I know. Open3Mod is free, open source, lightweight and just works well as a quick viewer of FBX and OBJ files. However, most of my models are BLEND files, which are not supported. Thankfully, Blender, the source editor of BLEND files, is also free, open source, somewhat lightweight and can load fairly quickly. Open3Mod's biggest flaw is that it does not support older FBX formats and gets confused sometimes about textures when opening OBJ files and DAE files.
3D Modeling and Animation Suite
Blender is awesome. I use it for the bulk of my workflow when working with 3D models. I highly recommend Blender to all 3D beginners but warn that it can be difficult to learn. Intermediate and advanced 3D modelers may already be comfortable with their toolset, but Blender is worth trying even if you have access to the most expensive solutions. It is simply one of the best all-in-one solutions I ever used.
Renderers are a dime a dozen, but it is difficult to find a good one that is easy enough to integrate with Blender. Thankfully, Blender has a fairly robust rendering system called Cycles. Cycles is fairly new but is integrated well with Blender without need of any additional setup. It comes with Blender. It's not the prettiest renderer and can be very slow to render even the simplest scene depending on your lighting setup. However, Cycles is extremely simple to work with and can be used for very advanced lighting setups.
Visualization and Prototyping
I use 3D scenes a lot when working on illustrations. Visualizing the scene before I actually draw it can be a good technique for working out the size and position of various elements of an illustration. I use Blender for still scenes and advanced lighting but switch to Unity when I want to work on particles like flames and animated or dynamic lighting. I also like to use Unity for script-based or procedural art, like fractal textures. I'm sure there are cheaper options out there, so I suggest you search around for your own preferred solution, but Unity works well for me.
Sculpting and Retopology
I admit I don't have access to the tools I want for sculpting. I want 3D Coat or ZBrush, but they are both too expensive for me right now. Instead, I use Blender's built-in sculpting mode, which is actually pretty robust and useful. I don't sculpt much, because I almost never have need for high-poly detailed meshes. If I had to really get into sculpting, I would consider getting ZBrush but would probably go with 3D Coat just because I can try it before I buy it.
Retopology is a term used to describe the act of changing the topology or the make-up of a model so it can have either fewer artifacts during animation or have smoother texturing. It is also used to improve the triangle count of models which may have too many triangles due to dynamic sculpting or just due to video game or application performance restrictions. I find that a model can look good even at an extremely low triangle count if the topology is well thought out and the textures are applied creatively. That said, sometimes, high triangle counts are necessary, especially for animated organic creatures. It is a challenge to make models low-poly yet good-looking.
My tool of choice for retopology is Blender's retopology plug-in, Contours. It is open source but a little difficult to get working inside Blender unless you want to pay for it. Regardless of the setup woes, it is a very good tool for saving time when retopologizing a mesh. You can also use Blender's sculpting mode to raise and lower the triangle count dynamically, but I find Contours much easier to work with.