From Second Life Wiki
Blender is a free open source 3D computer graphics program, available under the GNU General Public License for the Linux, Mac OS X, and Microsoft Windows operating systems.
Blender's features include 3D modeling, UV unwrapping, texturing, skeletal animation, water and smoke simulations, computer animation, rendering, computer simulation, video editing, and the ability to create interactive 3D applications, video games, animated film and visual effects. More advanced tools include rigid, realistic body, fluid, cloth and softbody dynamics simulation, modifier-based modeling, character animation, a node-based material and compositing system, and embedded scripting in Python.
There are currently two versions of Blender that people are using for Second Life. This unfortunately does occasionally lead to confusion about tool usage and placement.
Blender 2.49 has been criticized as being hard to learn and not up to date with the needs of the graphics artist community in general. Most people who are solely interested in sculpts use this version because of the add-ons available to make sculpt-making easier.
Blender 2.5x is the result of an effort by the Blender community to update the interface of Blender as well as to make it more feature rich. This version has recently reached stable status but still lacks a few features that make it a home run. Nevertheless new users are adopting it because of the easier to learn interface and the new add-on tools designed for Second Life that are coming online for this version.
For the purposes of Second Life Blender can allow a user to design and create the files necessary to import sculpties as well as the "mesh" object type. Blender can also export animations to the .BVH file format that Second Life uses.
The user base forms the core of the support mechanisms for Blender. Most users learn Blender through community tutorials and discussion forums on the internet, such as Blender Artists .
There are many inworld Second Life user groups (in numerous languages) that are dedicated to helping and teaching Blender users. Simply search for Blender in groups.
Numerous other sites, for example BlenderArt Magazine — a free, downloadable magazine with each issue handling a particular area in 3D development — and BlenderNation, provide information on everything surrounding Blender, showcase new techniques and features, and provide tutorials and other guides.
Inworld User groups
There are many add-ons for Blender that allow an easier content creation experience. To find add-ons that are specific to the needs of Second Life, look here:
Note: Add-ons for Blender 2.49 will not work on Blender 2.5x, so be sure of which version you are using.
Blender 2.49 specific
http://www.youtube.com/user/BlenderSL — BlenderSL on YouTube
http://blog.machinimatrix.org/video-tutorials/ — Machinimatrix
http://www.youtube.com/user/super3boy (Has tutorials for old and new Blender, so if you see an interface that doesn't look right, look at one of his other tutorials.)
http://www.youtube.com/watch?v=m5DwdgVbiXc&feature=BFa&list=PL11D6E99BB51A2BB2&lf=plpp Making of rigged clothing
Blender 2.5x specific
http://www.youtube.com/user/super3boy (has been updating his tutorials for 2.5)
http://www.lynda.com/ (this site is not free.)
http://en.wikibooks.org/wiki/Blender_3D:_Noob_to_Pro (This is currently being updated so your milage may vary)
Blender 2.5x and 2.6x
Nalates' Tutorials Index Clothes via Blender - Several tutorials on how Blender and SL work.
Second Life Shape Export - 2013 - This tutorial explains the problems with OBJ verses XML shape export. A method for getting OBJ shapes the correct size is shown.
3DBuzz Training Series
Blender 101, 102, and modeling a Viper Mk VII: Buzz Training Blender