I'd like to see some online vs. offline image optimizers comparison numbers, namely Yahoo! Smush.it vs. OptiPNG or pngcrush.
How those things differ in speed and resulting image size, and what is the best choice?
Very detailed and comprehensive comparison — with lots of tools and results on many different types of PNGs and optimizations:
http://css-ig.net/png-tools-overview
I think it's a much better source than PunyPNG's small comparison showing that their tool is best [partly at converting image formats rather than optimizing existing format] :)