Is there an equivalent to Google Sketchup?

The Art of illusion

Art of Illusion is a free, open source 3D modelling and rendering studio. Many of its capabilities rival those found in commercial programs. Highlights include subdivision surface based modelling tools, skeleton based animation, and a graphical language for designing procedural textures and materials..

K-3D (Available in the software center )

K-3D's interface uses your platform's look-and-feel, and it's consistent with the applications you already know. New artists will find K-3D easy to understand, and professionals feel right at home. K-3D is intuitive, consistent, and discoverable.

For a more advanced program you can try

Blender (Available in the software center)

Blender is a free open source 3D graphics application, available under the GNU General Public License for the Linux, Mac OS X, FreeBSD, OpenBSD and Microsoft Windows operating systems.

Blender's features include 3D modeling, UV unwrapping, texturing, rigging, water and smoke simulations, skinning, animating, rendering, particle and other simulations, non-linear editing, compositing, and the ability to create interactive 3D applications, video games, animated film, or visual effects. More advanced tools include rigid, realistic body, fluid, cloth and softbody dynamics simulation, modifier-based modeling, character animation, a node-based material and compositing system, and embedded scripting in Python.

Check for FreeCAD http://www.freecadweb.org/

"What is FreeCAD?

FreeCAD is a parametric 3D modeler. Parametric modeling allows you to easily modify your design by going back into your model history and changing its parameters. FreeCAD is open source (LGPL license) and completely modular, allowing for very advanced extension and customization."

Blender

Blender is the free open source 3D content creation suite, available for all major operating systems under the GNU General Public License.

Blender's features include 3D modeling, UV unwrapping, texturing, rigging, water and smoke simulations, skinning, animating, rendering, particle and other simulations, non-linear editing, compositing, and the ability to create interactive 3D applications, video games, animated film, or visual effects. More advanced tools include rigid, realistic body, fluid, cloth and softbody dynamics simulation, modifier-based modeling, character animation, a node-based material and compositing system, and embedded scripting in Python.