There are a lot of open source tools to optimize images that way – amost one for each image format. Working through all of them can be a drag. This is why Stoyan and Nicole took their knowledge, dedication and all of these tools and built one application that does all the optimizations for you in one go:
You can upload a bunch of images, give it a URL or use it as a Firefox extension or bookmarklet. Smushit will show you how many bytes you can save by removing cruft from the images and gives you all the images as a zip file to replace them on your site. How cool and easy is that?
Here’s a video of Stoyan and Nicole presenting Smushit.com at The Ajax Experience in Boston (sorry about the audio):