Simple as it is, a hammer is hugely important. Any fortifications you build will need to be nailed, and it's really impractical to use anything that wasn't meant for the job. Nailing two-by-fours up over windows has proven in many zombie apocalypses to actually help a lot when it comes to preventing zombies from breaking through. Even just a few of them makes a huge difference, because each piece increases the physical force needed to break through. In a pinch, a hammer is also a solid weapon for zombie killin', aside from the short range that puts you too close to the zombies for comfort.