Unix Command Line Tools for Windows
I work on Mac, Linux, and Windows machines during the course of an average day. A lot of this work is done on the command line. Since underneath a Mac is the heart of a Linux type machine, the basic commands that work on one also work on the other. Windows machines are a different story. Lots of commands that work on Mac/Linux are different on Windows machines or simply don't exist at all.
For example, when you want to see what's in a directory from the command line the Mac/Linux command is "ls", but on Windows, it's "dir". Since most of my time is spent with Mac/Linux I end up trying to use "ls" frequently when I'm working on a Windows machine. This doesn't hurt anything, but it doesn't work and it is a little break in the concentration.
The good news is that since most of the command line tools on a Linux machine are open source, they can be ported to different operating systems. This is exactly what was done with the Open Source project UnxUtils. It's a collection of most of the key command line tools Linux folks are used to working with. If you spend most of your time on Mac/Linux but have to bounce around the Windows command line on occasion, I highly recommend them.