The dramatic sand dunes at Sossusvlei can be visited at any time of year but the dry May to October winter months are widely regarded as the best time to visit Namibia for game viewing - especially in Etosha National Park. The skies are clear, the risk of malaria is at its lowest, and animals are increasingly concentrated around water sources as the dry season wears on. Prepare however for freezing night time temperatures in June and July whereas September and October can be extremely hot.
The summer rains between November and April turn the country green and occasionally bring floods to the desert. December can see very rewarding game viewing in Etosha and summer is the best time to go to Namibia for bird watching though you'll need to watch out for malaria in the Caprivi Strip (newly renamed the Zambezi Region), Namibia's wettest region and a place best avoided in the peak wet months of January and February.