It depends what you are trying to improve on. [link] Is very good for human anatomy. The models are very anatomical, they show the muscles, rather than skin, what is what you are most likely to find if you search stock images. What I like about this site is that you can rotate the subjects left & right to view them from different angles, it is insanely helpful in getting the desired view. I also like that the 3D models are consistant. What I mean by that is the female model is the same female model for every female stock image. I always think it is important for references to remain consistant, that way you don't have to distort your stock images.
[link] This is the same website, just a subsection. You can rotate these models freely. Most of these are hands, which are enormously helpful as references.
TheWalkingGhost already touched upon Goggle Sketchup [link] That software is helpful for building references. You can go to their page and download building models. They are really helpful, either as references, or something to make part of your work. You can manipulate the building to fit your image, extract the file as line art and then put that image in your piece. The thing I don't like is you need a licence to purchase it ($495) to unlock the trial version. Oh, and the trial version only lasts for 8 hours. Yep, that's right. 8 hours.
Blender is good if you can work around the interface and the steep learning curve. I wanted to dabble in 3D modeling and looked for a basic modeling program that would let me learn the basics. So I found Metasequoia. It doesn't have the bells and whistles like Blender but it is a solid program that doesn't require a lot of your system. And you can make some pretty nice models with it. What you learn with it will help you when you go to a more complex program. It is a step up from Google Sketchup. Find the freeware version at the link. The full version with more features can be purchased for $45. [link]