Non-Employee Network Access

I was hoping to start a discussion on best pratices for security concerns related to non-employees accessing a companies network resources while on-site. Vendors, partners, guests, etc. often require some level of network access when on-site at a company, whether it be Internet or some internal resource. The potential for data leakage is a concern as well as exposing the network to potentially poorly protected laptops. How do we let these people get to the resources they need, and yet ensure the security of the network. What types of policies are out there, and what technology is employed to enforce them?