We've heard a lot about optimizing CSS, HTML and JavaScript but one thing that is less talked about is how much extra information image editors put into image files. You might think you've done a great job optimizing your GIFs, PNGs and JPGs while still keeping them visually pleasing but when you use a text editor you'll realize that there is quite a big amount of data you can save by removing information about the image editor used, the date the file was edited last and lots of other bits that really are redundant.
There are a lot of free tools that strip this information from the files for you and squeeze some extra optimization out of the file without affecting the look. The problem is that all of them are command-line based and you need to know how to use them. Stoyan Stefanov and Nicole Sullivan of the Yahoo exceptional performance team took all of these tools and their experience in using them and built one application that does all the optimizations for you in one go:
You can upload images, give it a URL or use smushit 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.
Here's a video of Stoyan and Nicole presenting Smushit.com at The Ajax Experience in Boston (sorry about the audio):