A bench block is an essential tool to have if you plan on working on your guns even occasionally. There are numerous options to choose from, but the ones that we use in the shop are these from Brownells.com. They are 4″ disks of semi-hard plastic that will generally not damage the finish of your gun and provide a stable surface for driving pins and turning screws. They come with through holes for pins to clear, and v-slots to help to hold your work steady.
These blocks are available in two versions, the blue is for pistols and the green is for rifles and shotguns. The only real difference between the two is the arrangement of the through holes and the addition of a second slot on the handgun version. Functionally, either version will work just fine for most all situations regardless of what it is intended for. In some cases, there are blocks available that are designed for a specific firearm (such as the AR-15), but in most all other cases these general use tools are an excellent addition to your workbench.
Overall, a bench block is one of the best ways to protect your guns from damage while working on them. They retail for 19.99, and are easily worth the price even if you save yourself from scratching the finish on just one gun.