I created custom hands for my human-robot interaction work with the Rethink Robotics Baxter Research Robot. These hands are great for safe and comfortable hand-to-hand contact with Baxter. (And even if you don't have a Baxter robot, they can make an entertaining desk ornament or device for playful roughhousing.) The attached STL files contain a front and back piece for each of Baxter's hands. The pieces can be assembled using the protips below.
To attach the 3D-printed hands to the parallel rails with threaded holes on Baxter's wrist (as shown in the included picture of Baxter's wrist), you'll need two M4x0.7 screws per hand. The palm parts of the hand have holes on the back for standoffs (they'll press fit in if you print the hand on a makerbot, or you can glue them in if you print the hand on a higher quality 3D printer). Once the "palm" part of the hand is on the robot, you can put screws through the holes in the back-of-hand part and secure them in these standoffs to close up the hand.
I also typically fill the depressions in the hand with EcoFlex 10 silicone. To do this, I usually roll some modeling clay into a long cylinder and outline the outer edge of the hand with clay. Then I mold the silicone directly in the hand by pouring it into the indent until it is flush with the highest point of the clay. I do this on the front of the palm and the back of the back of the hand.
I've printed this part on several different settings using a MakerBot Replicator 2. For speed purposes, I recommend printing with the lowest resolution. Infill thinking is similar, although it depends on how strong you need the hand to be. The part also prints great on a Dimension and Objet with standard settings.