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              If you are basically a Ubuntu user who likes to be on ubuntu more than windows [ like me ] then you can try many other linux distros which can be ubuntu alternatives . ubuntu as you know is offered for download as an ISO image file as well as can be requested free [ Free CD's ] . However, there are plenty of versions to choose from !

            The Ubuntu project has many projects . You can observe from the fact that they daily provide updates worth 10 . If you go on official home page of ubuntu , you will see many other ubuntu like linux versions each adapting the main Ubuntu release by adding / removing soft-ware, mostly in the form of some other unique algorithm , working and desktop environment
Here are the all the best linux distributions [ Free ] which can be ubuntu alternatives :
 
Kubuntu :
This is a version of Ubuntu that uses the KDE desktop environment  instead of Gnome. KDE is best alternative for ubuntu according to me as kubuntu and ubuntu have much similar working and features they offer are similar . You can again request free CD .
Link

Edubuntu :
This linux OS focuses on  educational software and a child-friendly                      user-interface. Mostly preferralve for  educational environments.
Link

MythBuntu : 
Mythbuntu is a community supported add-on for Ubuntu focused upon setting up a standalone MythTV based PVR system. It can be used to prepare a standalone system or for integration with an existing MythTV network.
Link

Xubuntu :
Based on the Xfce desktop environment . This is a stripped-down desktop
designed to run optimally on all types of hardware mostly for very very old p3 computers or still ancient than that one ! Features are almost same with just little less attractive and less chic type .
 Link


Fedora :
This is one of the best alternative after kubuntu and again many people use it . I have to try it yet will update more after my trial pack
Link

Choosing a release
         Every distros of linux are released every six months and every two years a long-term support (LTS) release is made . If you have got the linux bug then i suggest you try all the free versions and even explorer the paid ones like redhat !