If you're using Photoshop to create images for the web and/or are interested in squeezing out every useless KB you can, I came across a tip at PhotoshopSupport.com that can save you up to 70% in file size. Go into the "File Handling" section of Photoshop's preferences and change the the 'Image Previews' option from the default "always save" to "never save" or "always ask." Image previews are what allows you to see the image in icon form when browsing through your system's folders, whether on OS X or Windows. Image previews also take up a decent share of extra file size you might not always need or want in your files, so changing that feature will allow you to really streamline your site's images, or at the least reclaim some of your hard drive.