The Blender exporter lets you publish 3D models straight from Blender to Sketchfab.
As of Blender 2.70, the Sketchfab exporter is natively integrated! As long as you have the latest version, you can already export your models directly from Blender.
When the upload is complete, you can view your model on Sketchfab, adjust rendering settings, and save the default view.
For a more in-depth guide to this process, including texture and shading processing in both Blender and Sketchfab, check out Phil Gosh's awesome tutorial and visit the blog.
If you prefer to use an older version of Blender, you can still manually install the addon.