WinMerge is an excellent open source tool for comparing two versions on text file. Unfortunately, it runs only on MS Windows platform. As a Linux user, I was looking for similar tools which could be run in Linux natively. The following two application can to a job of WinMerge. If you running Ubuntu Linux, they could easily installed using package manager.

Kdiff3

http://kdiff3.sourceforge.net/

Allows you to compare file and directories. Has nice and intuitive interface.

Meld

http://meld.sourceforge.net/

Another tool offering similar functionality. It also has intergration with CVS source control.