There's a couple of them floating around:
XNA Sculpt Preview (Needs xna installed)
http://www.jhurliman.org/download/sc...view-1.0.0.zip
Cel Sculptpreview
http://www.xs4all.nl/~elout/sculptie/
Also, when you upload to SL, you can preview in the upload window by changing the image type to sculpted in the pull-down. The window is tiny, but it's the most accurate preview as it's the actual rendering engine of course.