Sky viewers and planetaria

Stellarium
Planetarium software that shows a 3D simulation of the night sky (requires installation)

WorldWide Telescope
American Astronomical Society’s WorldWide Telescope project. View the sky online or install the viewer for more options. Resources and information also available; in particular, see the Guided Tours and Introductory Astronomy Sections.

Virtual Sky
VirtualSky is a browser-based planetarium from Las Cumbres Observatory that lets you see what is visible in the sky from any location on Earth. It can be customized and included on your own website, blog etc. It is provided freely for educational and non-profit use.