AlternativeTo.net is a great resource for any computer user. The site lists alterantives to popular software applications. Many of the alternatives are free, some are not. The great thing about the site is that it lists things by category so it is easy to find an alternative to what you are using now.
Maybe you want a free version of something, such as replacing Microsoft Office with OpenOffice, or you just want to try some different types of software. Maybe the software you are using now doesn't do everything you need it to do. Whatever the case, AlternativeTo.net can help.
The listings are very comprehensive and include a great description. You can sort results by tag, license, or platform. They have alternatives for Windows, Mac, Linux, and even for mobile applications.
Below is a listing of alternatives to Microsoft Office: