For simple resizing of your existing Windows partition, Norm has done a good job in showing you how to get it done with Easus. If you want to do more, such as creating new partitions of different file format (such as Ext3 or HFS), then Gnome Partition Editor (Gparted) is the one for you.

GParted is a powerful application that you can use to change the partition organization therein, while preserving the partition contents. It allows you to resize, move, create and format your partition to various file formats. The file formats that it supports include ext2, ext3, fat16, fat32, hfs, hfs+, jfs, linux-swap, ntfs, reiser4, reiserfs, ufs and xfs. While most partition management software are restricted to a particular platform, GParted is cross-platform compatible and can be used regardless you are using Windows, Mac or Linux.