The Google 3D Warehouse has collections of 3D models grouped by city. I find this useful, as I can browse models by location. You can view many of these models in both Google Earth and Google SketchUp. I am partial to the model of Fenway Park, home to my beloved Boston Red Sox, but there are a lot great user-contributed items.
If you want to view what is available in any particular area as you navigate in Google Earth, be sure to download the 3D Warehouse network link. Once you have done this, house-shaped icons appear in Google Earth that indicate where 3D models are located.
If you want to create your own models in Google SketchUp that you can add to the 3D Warehouse or just share with your friends, be sure to check out these excellent tutorials.