Another indispensable tool in the arsenal of textile artists is the needle - tiny in size but with the power to reduce you to tears and/or cause you to say things that would merit having your mouth washed out with soap...
A Guide to Sewing Machine Needle Selection
All About Sewing Machine Needles
Anatomy of a Needle
Dull Needle Illustration - many machine problems can be corrected by simply changing the needle - through the use of magnification, this illustration demonstrates why http://www.a1sewingmachine.com/needles.htm
Embroidery Needle Reference Chart
Embroidery Needles: How to Select the Right Needle for Your Project; and Easy Steps for Choosing the Correct Needles for Today's Needlework.
Fabric/Needle Cross Reference
Hand Sewing Needle Guide
How’s Your Needle Knowledge? – this excellent video takes you through all of the basics about needles from detailed information about the parts of a needle to the differences between the European & American sizes to different needle types to how to choose the right needle for your project. Even if you're experienced, you'll probably learn something.
Needle Know-How: Guidelines for type and size of needle for different weights and types of fabrics
Needle Selection Guide
Needle Size Chart (European/US)
Quilt Tips [about needles] from Quilters Around the World
Schmetz Sewing Machine Needle Guide - illustrated
Troubleshooting Sewing Problems
Twin Needle Reference Chart
What Every Home Sewer Needs to Know
Using the Right Needle