I've mentioned it several times in this thread. Aside from being the only D&D supplement I know of that's been really explored a psuedo-African setting, it's also pretty well done. It's positive in that it's actually rather neutral in how it approaches an African fantasy setting, somewhat similar to how D&D approaches European fantasy. Nyambe is a land with a variety of religions (not just a layman's understanding of vodun), variety of ethnic groups (not all Yoruba/Zuluu/Masai hybrid), variety of terrain (not all jungle/savannah), variety of languages (not generic mock whatever), a variety of cultures and societies. It's not an idealistic natural paradise or a dark and unexplored disease ridden land of savage danger. It's a land of fantasy adventure that just happens to draw on Africa as opposed to Europe or Asia for inspiration- and still draws from other cultures.
I also highly recommend Spears of the Dawn, although it's OSR rather than d20. It's a smaller setting -mostly influenced by west African and nearby regions- but has some cool ideas.