For you new and more experienced designers, try out the SLCP offline 3D viewer! While you can't make changes, it allows you to upload texture maps for clothing and skins to preview them before uploading to SL! Banshee Studios has released an updated version that you can download at this link:
http://www.bansheestudios.com/files/slcp_win.zip. I've enclosed a screenshot of the application so you can see the tool:

This is the screen without any of your textures added, It can do both male and female bodies, and change the background color if needed. You have the ability to fly around and zoom on the image. This is great to check for errors or image adjustments! I have GIMP open in one window and SLCP that way I can go back to GIMP and make any changes and reload the final image after saving. If you have any other tools, post a comment and a review!