How compare or diff files

Notepad++:

Menu bar => Plugins => Compare => Compare
UltraEdit:
Menu bar => File => Compare or Alt+F11
DOS prompt:
ASCII file
fc filename1 filename2
Binary file
fc /b filename1 filename2
Linux/Unix:
cmp filename1 filename2
diff filename1 filename2
ClearCase:
cleardiff filename1 filename2
cleartool diff filename1 filename2