If you want to grab textures out of Second Life to see how people have made those neat trees, the stunning clothes, or maybe an old texture you lost, GLIntercept is the tool for the job. GLIntercept is a program that grabs OpenGL information directly from memory. It does a lot more than just capturing textures; with it you can extract prim information, avatars, and textures. This information can be exported into other 3D programs, such as Maya. This hack, however, is just about the images.
To get started, you'll first need to download GLIntercept (http://glintercept.nutty.org/download.html) from its website. They have several versions, but if you're mostly interested in grabbing textures, just grab the latest Binary for Windows.
Once it's downloaded, run the installation program. A standard installation will put the files on your C: drive, in the C:\Program Files\GLIntercept0_5 directory -- "0_5" will likely vary depending on the version you download. There is no executable GLIntercept program, so don't worry because you can't find an *.exe file.
Next, copy the files opengl32.dll and gliConfig.ini from the GLIntercept directory into your Second Life directory (for example: C:\Program Files\SecondLife). Once those files are copied, run the SecondLife.exe program; use it for a few minutes, then log out.
NOTE: Second Life will operate a good bit slower with GLIntercept. You may want to log in a head of time, get where you want to grab textures, and then log out.
There will be two new files and three new folders in your Second Life Directory. The folder named Images is what you want. Within Images will be several hundred, maybe several thousand, JPG and PNG files. These are all the textures that your Second Life rendered on screen while you were logged in.
NOTE: Second Life images appear to load in stages. To get a full texture, you need to wait until a prim and its texture(s) have fully loaded before moving off or logging out. Otherwise all you may get of a 512x512 texture is a smaller, 128x128 version.