Smushit.com is a service that goes beyond the limitations of Photoshop, Fireworks & Co. It uses image format specific non-lossy image optimization tools to squeeze the last bytes out of your images - without changing their look or visual quality. You'll get a report of how many bytes you can save by optimizing your images and all the changed images as a single zip for download.
Mind-mapping.org has a good capability for selecting software by map type and operating system (click on the 'Refine software list' tab at the top right of its web page), and shows the price. Unfortunately, it doesn't allow you to filter on price or choose to see only the free ones. This article is therefore a quick list to current software that is free or has a free option. It excludes applications that have a time limit on their operation, for example, free trial software or web applications.