Simple Sample Theme - (Download Zip File) 

"The sample theme that Sony released makes it MUCH HARDER to customize than it should be.  The icon filenames don't describe the icons and the XML file is confusing.  Because of this, I created an EASY TO USE template for people to use instead of Sony's sample theme." -iRockHard

Theme Creation Video (YouTube Link)
By rubixcube6

This tool is really great! It's basically a little program you can run that will show you how your theme will look and will also compile it all for you. You can pick your colors, what icons you want, everything, and you don't need to edit the xml file by hand. 

Photoshop Custom Shape Icons (Link)
By Matu 

This is a whole set of VECTOR custom shapes of all the PS3 XMB icons. If you use Photoshop this is a must have. To install, grab the file from his site, then unzip. From within photoshop go to your shape tool, click on custom shape, then the arrow in the upper right, then load shapes. (just look at this picture, it's easier than explaining!) 

PS3 Theme Extractor (Link)
By codelogic

This little program will let you extract all the files from a PS3 theme, includings wallpapers and icons. To use, right-click on a PS3 theme file and choose "Open with", and then find the pstextractor.exe on your hard drive.

NOTE: Some theme creators likely won't appreciate you stealing their icons and wallpapers (which can take a LOT of time to create) for your own themes so be sure to ALWAYS ask first. It's just more polite.

"This application has failed to start because MSVCR71.dll was not found."

I ran into this problem when I tried to compile my theme. To fix this grab these two files and drop them into your GimConv folder. (unzip them, of course!)

File 1
File 2

 *Note, you can also put them in your windows/system32 folder