What is the best, free, grep application for use with Windows instead of Linux?
Either use Grep for Windows or the findstr command suggested by Moron.
http://unxutils.sourceforge.net/ has a bunch of GNU tools ported to Windows.
Newer versions of Windows include a built-in grep, called findstr. It'll do just about whatever you need. For example
C:\>findstr /I windows c:\boot.ini default=multi(0)disk(0)rdisk(0)partition(1)\WINDOWS multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professio nal" /fastdetect /NOEXECUTE=ALWAYSON
For more info, just run findstr /? on a command prompt.
I know this has been dormant, but I don't see very many options listed. I just came across AstroGrep referenced by twlichty at How to search inside files... and it works fine for me. As to "best", I think that is subjective, but I prefer command line grep and this is a GUI tool, but my preference is not enough to keep me looking...
FART provides some of the functionality of grep, depends on your requirements but I recommend you take a look: