A shortening of Different Horses for Different Courses.
This saying means that different tools can be better for different jobs. I find myself using this saying when confronted with language or platform "discussions" that are rapidly descending into flame wars. You wouldn't use a screwdriver to hammer a nail in, nor would you use a lawnmower to trim a hedge.
My most often used arguments are: "OK, use Linux for the server, and use win2k for the desktop. Horses for Courses dude." and "Why are you using C++ to write a web parser? Hack it together with Perl. Horses for Courses and all that."