Stellarium is a freeware alternative to planetarium software such as Starry Night.  It offers many of the capabilities of other software on the market.  You can view star charts of the night sky from any location on earth at any time.  Some telescopic images of select objects are included, but the images are not nearly as complete as in the Microsoft World Telescope shown below.  If you are someone who is looking for a free way to learn your way around the night sky and then observe these objects with a telescope, Stellarium is one of your better options.

Google Sky is an online free service provided by Google which offers basic planetarium software services without a download.  Google Sky is a good alternative for those who wish to navigate the sky on the computer, but it lacks features that planetarium software provide.  Google incorporates a similar service into it's free download, Google Earth, if you wish to take Google Sky out of your web browser.

Wikisky is another online free service similar to Google Sky which allows the user to view the locations of stellar objects through their web browser.  There is no download, but constant internet connection is required.  If you don't want to install a program on your computer yet and have a good internet connection, then Google Sky, Wikisky, or another similar service, are all good options.  But installed software generally offers more options.

Microsoft World Telescope is a free program developed by Microsoft, which incorporates images from multiple telescopes into a planetarium software package.  The user can then create, or view other users', tours of the night sky.  The program itself can be downloaded and installed on your computer, but the telescope images are downloaded as you view them.  Therefore, many features become unusable without a high-speed internet connection.  This program offers the most in terms of visually impressive photographs of distant objects, but might not be the best option for someone who wishes to find the objects themselves.

Celestia is a free 3D space simulation program.  Although it lacks some features to navigate the sky from your particular location, its ability to allow the user to 'travel' to stellar objects and view the night sky from that location, is quite impressive.