Available via Cygwin, Babun, GOW, msys.
Ranked in these QuestionsQuestion Ranking
Pro Great for people used to *NIX systems
Since it's pretty much the same grep used on UNIX systems, people who have used grep on any *NIX system will find themselves at home with the Windows version.
Pro It handles files of any size
At least in Linux.
Con Cannot handle files
When searching through files with 10MB or more it starts having issues and even crashes depending on the file size and the machine it's running on.