HDPrepar
The Xbox is compatible with a number of external hard drives, but the drives aren't always readily compatible with the Xbox. This is why programs like HDPrepar exist; they prepare the hard drives for use with an Xbox, writing the partitions and basic lines of code needed to function with the system.
Xplorer360
The Xbox 360 console can access videos and music files on your computer, and you can access the 360's files on your computer. However, if you want to see the other files that are hidden in your Xbox's hard drive, you will need something like Xplorer 360, a program that lets you explore the hard drive in the same explorer window you use to explore your Windows PC's files.
HDDhackr
If you use a Western Digital hard drive and want to connect it to your console, you may have difficulty without an application like HDDhackr, which is designed solely for getting a Western Digital brand HDD onto an Xbox 360.
wxHdd
A program like Xplorer functions as a Windows Explorer window for the Xbox 360, but wxHdd also includes a file viewer that lets you look at the image files stored in your Xbox -- which means you can use some of the images stored in theme files on your computer.