This is a handy tool provided for us from Spinwave. We use it here all the time. (You may wish to bookmark this page.)
Some of our page loading times have been cut in half simply by crunching our gifs with the Gif Cruncher. Don't be fooled by downloading gifs from the Internet and thinking they're already optimized. I hazard a guess that more than 95% of them are NOT optimized for your web pages.
Although .gif is the most popular format of graphics you find on the Internet, just as popular is the .jpeg format you find used mostly for photos and photo artwork. Until recently, free online jpeg optimizers were hard to find. Click here to optimize your jpegs free here now.