Planetarium is described as 'A browser-based virtual planetarium of stars and planets customizable by location (latitude and longitude) and time of day' and is an website in the Education & Reference category. There are more than 25 alternatives to Planetarium, not only websites but also apps for a variety of platforms, including iPhone, iPad, Windows, Android and Android Tablet. The best alternative is Google Earth. It's not free, so if you're looking for a free alternative, you could try Stellarium or Celestia. Other great sites and apps similar to Planetarium are Sky Map (Free, Open Source), Gaia Sky (Free, Open Source), NASA World Wind (Free, Open Source) and KStars (Free, Open Source).
The Starry Night apps are the most comprehensive educational astronomy apps. Use it with your introductory astronomy textbook or on its own. It's a powerful, portable planetarium that fits in your backpack. It puts thousands of stars and planets at your fingertips.
Mitaka is software for visualizing the known Universe with up-to-date observational data and theoretical models. Mitaka users can seamlessly navigate through space, from the Earth to the edges of the known Universe.