The Namibia Dollar (N$ or NAD) is the local currency which is linked with the South African Rand (ZAR) and is exactly the same value/exchange rate (fixed at a rate of 1:1). South African Rands can be used freely throughout Namibia, however Namibian Dollars are not accepted in South Africa. Even though the US$ is widely used in many countries, most rural Namibians would not want to receive US$ currency as it can be a problem to exchange in the smaller towns. If you are also going to be visiting Botswana and Zimbabwe on your trip, be sure to have Botswana Pula (BWP) with you at the border crossing into Botswana and US$ for the rest as well as for use in Zimbabwe.
