MilkShape 3D
If you're looking for an easy-to-learn mesh program with basic capabilities, try this one to learn with. You can create models from scratch or edit existing files you've exported from the Sims 2. When your model is ready for use in-game, Milkshape allows you to export the project in the correct file type. If you're unfamiliar with mesh-editing programs, Milkshape's simplicity makes it good for beginners, and it's also shareware.
The MTS2 Mesh Tool
The MTS2 Mesh tool was created by Sims fans specifically for the Sims 2 game. It allows you to edit existing Sims 2 meshes in more advanced ways than with Bodyshop, but does not allow the creation of completely new meshes. It works with OBJ files only and requires that you use SimPE. It is not a 3D polygon editor such as MilkShape 3D or Blender.
Blender
Blender is a program similar to MilkShape, though it offers more abilities for advanced users in a less intuitive interface. It allows you to do 3D model creation, skinning, texturing and animation. For Sims 2 objects, you'll need to convert your work before exporting them back into the game. Blender is a free, open-source program.
SimPE
SimPE isn't just a mesh-editing program for the Sims 2. It edits game files in a variety of ways, allowing you to mod character attributes, neighborhood files and object meshes. If you use a 3D modeling program to create a new mesh, sometimes you need to convert the resulting file to a format that will work in your Sims 2 game, and SimPE makes that possible. The program is free.