This is my host spotlight on the SitePoint Podcast #24 coming out this Friday, but I liked it so much I wanted to share it here as well.
punypng lets you upload a bunch of JPG, GIF, or PNG images (up to 500KB each) and it uses various tricks to reduce their size without any loss in quality. In the case of PNGs with transparency, it uses a neat trick called "dirty transparency" whereby it overwrites the image data stored for transparent parts of the image with a solid color, which compresses better. In the case of JPEGs, it event tests if the file could be better stored as a PNG, and lets you know as much. When it’s done, it presents a simple report showing the amount of space it was able to save. Savings of 30% are not uncommon.
The only annoying part for me is that this is a web service. I kind of wish I had this as an application on my Mac that I could just drag files onto. The option to download your compressed files as a single ZIP file is nice, but it’s still a little inconvenient.