Windows and Linux Tool to compare and merge files

Windows :

1) Winmerge : http://winmerge.org/
     Freeware.

2) Araxis : 30 days trial period

http://www.araxis.com/merge/

Linux :

1) Meld:  freeware
     http://meldmerge.org/ 

2) Guiffy : http://www.guiffy.com/
    21 days trial period.

3) Kdiff3 : http://sourceforge.net/projects/kdiff3/
     freeware and good to use.