Tools

Blender
A free open source 3D content creation suite.

QAvimator
A free animation editor.

SculptyPaint
A free tool for generating 3D sculpted models for SecondLife.

The Gimp
The GIMP is the GNU Image Manipulation Program, a free alternative to Photoshop

Wood Workshop
A free texture editor that specializes in the creation of seamless wood textures.

General

Nicola Escher - Second Life tutorials
Tutorials on creating clothes, tattoos, and even a complete avatar.

Second Life Video Tutorials
The official Second Life Video Tutorials covering all aspects of using and creating in Second Life.

Building

Replicate a Real Life Building
Tutorial: How to Replicate a Real Life Building in Second Life.

Ultimate guide to prim twisting
Ayumi Cassini Does Second Life: The ultimate guide to prim twisting

Scripting

LSL Portal - Second Life Wiki
The official LL hosted wiki intended to provide accurate documentation for LSL, the Second Life scripting language. Includes examples and tutorials.

LSL Wiki - Second Life Wiki
The original LSL wiki, still active and provides some useful information not available on the official wiki.

LSL Particles System script generator
A web based particle system script generator

nand Nerd's Web-Based Particle System Interface
A web based particle system script generator that can directly updated an in-world object for previewing the particle effects

Sculpties

Sculpted Prims - Second Life Wiki
The definitive guide to sculpted prims for SL

Primstar - Blender Scripts for Second Life Sculpties
Primstar is a set of Blender scripts to enable fast and easy creation of Second Life content such as sculptured primitives.

The blender trail
A very good series of video tutorials for using Blender and the Primstar scripts to make sculpties. Starting from installing Blender to advanced techniques.

Amanda Levitsky - How to Make Sculpted Prims with Blender
How to make a sculpted vase using Blender.

Textures

CG Textures - The worlds largest free texture site
An awesome collection of free textures that you may use on items that you make and sell, but not free for you to sell the textures on their own or as part of a texture pack.

Texture Tools - Second Life Wiki
Resources to help you texture better in Second Life - Links to tools and tutorials.

Making Seamless Background Tiles
Examples & instructions for creating seamless tiles

Texture Alignment Pattern
A texture alignment pattern and tutorial on using it