Optimizing images for web usage

I want to make my website as quick to load as possible. For this I want my images as small as I can get them. For icons I have found using PNG images with a small palette and some dithering is quite effective. I discovered this through the save for web feature in Photoshop. Unfortunately I don't have access to Photoshop anymore...

So... does anyone know of some good software (preferably free) which can really compress images? Preferably I'd want some previewing fe开发者_C百科ature where you can tweak settings and see how they affect the size and the compressed image. What I want is pretty much the save for web feature in photoshop, without having go buy photoshop and go broke :p

---

Check out Smush.it.

---

pngquant generates PNG-8 better than Photoshop's Save for Web.