Ah! My Goddess is a one-of-a-kind love story, especially how it almost seamlessly integrates so many different interests and ideas. Among these are references to Norse mythology, comedy, love, and racing.
A few anime and manga one may enjoy in addition to Ah! My Goddess...
If you're looking for a harem-type story (a story where a boy and a lot of females make up the main cast), there are a few well-known ones such as Urusei Yatsura, Tenchi Muyo, Onegai Teacher, and Wish.
Wish is the most similar to Ah! My Goddess since it's premise is about a man saving an angel, and the angel staying with the man in order to grant him a wish in exchange for saving its life. They are soon joined by a huge cast of other characters, but that's all a spoiler.
Onegai Teacher is also similar because it's about a student who marries his teacher... who happens to be an alien. Throughout the series, they are also visited by her little sister (who's just as bratty as Skuld, if not moreso) and her mother (who's just as voluptuous and horny as Urd, is not moreso). The teacher herself is very similar to Belldandy, and even shares the same voice actress! The last reason Ah! My Goddess fans may want to check this series out is because the character designer is Mr. Hiroaki Gohda, the director for anything animated for Ah! My Goddess.
Kosuke Fujishima is also friends with Naoko Takeuchi, who is the creator of Bishoujo Senshi Sailormoon. The manga and the live-action show are worth a look, although the anime tends to be repetitive.
If you like upside-down faerie tales, the following are good: Ayashi no Ceres (Ceres: Celestial Legend), Shoujo Kakumei Utena (Revolutionary Girl Utena), Saikano, and Video Girl Ai.
All in all, there are a lot of things you could enjoy; it's all up to personal taste. Hopefully this small list can get you started for finding what you enjoy in addition to this series.
A few games you might enjoy...
Although out of print (despite being on the Greatest Hits list) is Xenogears, a game made by Monolith software and published by Square-Enix for the Playstation console. It's an RPG with heavy psychological, mythological, religious, and historical references including Norse mythology. Despite all that, it's an epic love story. Ah! My Goddess has a well-developed world and system, but at the heart of it all is the love story between two young individuals.