wait, there's some more.
First, one should know (as you presumably do, but for any case, I'll just mention it) that there are two basic approaches: raster ('pixel') - based, and vector based. Photoshop is actually the master for the first (Gimp, as mentioned, out now for windows as well, is freeware but maybe not quite as powerful), and it includes a few vector capacities as well; for the vector family, this isn't quite as simple. For flat illustrations, the top line products are freehand and illustrator, but also he actual 3D modelling tools are vector based. I don't know if you're looking for these, or just for 3D effects. 3D effects can be obtained with a number of plugins available for Photoshop, such as KPT ('Kai's Power Tools (or maybe Kais Power Tools)), EyeCandy, etc. (As far as I know, not all of them exist for Gimp.) If you are looking for actual 3D modelling, well, there's plenty, and they all fulfill different needs. There are things like Adobe Dimensions, Amapi Designer, etc. etc.; there's AutoCAD, and then the animation tools, at their high end Maya, provide prettey decent modelling capacities as well. All of these export images in noumerous formats, so you should always be able to work them in a flat vector or raster image application.
Also there are things like RasterVect or StreamLine, that vectorize raser images, which is useful for certain operations, eg. for using it whith vector tools like freehand or cad.
(I can come up with more details if needed.)