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There's a hammer, wrench and drill in the new gazebo models database: https://bitbucket.org/osrf/gazebo_models (wiki: http://gazebosim.org/wiki/Model_database)

For making models take a look at the beginner's section in the tutorials: http://gazebosim.org/wiki/Tutorials ("Make a Model", "Importing a Mesh" and maybe also "Building a World")

Let me ask you a question as well though: "it's just to do some grasping stuff with the NASA Robonaut r2 model (which runs just fine)"
You're able to perform manipulation with NASA's R2 model? The last time I tried the R2 model (and it hasn't changed since then) it was unusable for anything but looking at it. The impedance controller was not usable for any form of precise arm navigation and when trying to get the arm stack working it failed due to collisions in the collision model (there's a post on ros answers about the issue).