ZeroSprites is a CSS sprites generator aimed at area minimization using VLSI floorplaning algorithms.
Online version: http://cly.jsser.com/sprites/

Usage:
sprites.sh (directory | png files)
Output consists of sprites.txt and sprites.png

Notice:
Use pngnq to generate png8 output if necessary.
You can use png optimizer(optipng, pngout) to make output smaller.

Reference:
H. H. Chan and I. L. Markov, ``Practical Slicing and Non-slicing Block-Packing without Simulated Annealing", Technical Report CSE-TR-487-04, The University of Michigan, 2004.
T.-C. Chen and Y.-W. Chang, ``Modern Floorplanning Based on B*-trees and Fast Simulated Annealing", IEEE Trans. Computer-Aided Design of Integrated Circuits and Systems (TCAD), Vol. 25, No. 4, pp. 637--650, April 2006.