Softminer.net

1. using Notepad ++
to compare files using notepad++ you need to install compare plugin
download the plugin (.ddl) from http://sourceforge.net/projects/npp-plugins/files/ComparePlugin/
go to notepad++ and from Import -> import plugin
restart notepad++

2. DiffMerge is a free product from sourcegear
http://www.sourcegear.com/diffmerge/

3. TFS (Team Foundation System) in visual studio
right click on the file and then click on compare

4. if you dont have notepad++ installed you can use winmerge
http://winmerge.org/