I think that http://pngquant.org/ does the same technique.

Also see http://pngnq.sourceforge.net/index.html

There are several other image optimization tools incorporated into a OS X tool http://imageoptim.com/

You don't understand.

It must be a start-up with a freemium model. Soon they will reveal a monthly subscription option with a professional bulk uploader in Flash or even a desktop client. Just wait and see.

What would be wrong with that?

It's a pipe dream, but otherwise it's fine.

Ditto. I'd like to see some side-by-side comparisons with PNGOUT, PNGCrush, OptiPNG, and AdvPNG. I use ImageOptim on my Mac to automate my PNG optimization—and it can get some impressive gains.

On the Mac I use ImageAlpha to reduce the colours to 256 (it has a preview window so you can see if it's OK), then when saving the file I choose the option to send it to ImageOptim.

On Linux you can get the same by using pngquant, then using trimage. AFAIK there's no GUI frontend for pngquant so I guess how many colours would be ok and check before running trimage.

The reason why I use 2 tools is that ImageAlpha/pngquant is lossy and ImageOptim/trimage is lossless.

The tools you mention use lossless compression. For 24-bit PNG files that means they probably won't come close the kind of lossy compression used by TinyPNG.

The comparison will be subjective. One will retain full detail (24-bit color) and the other will not. The "better" will be subjective, as I said.

I wish Photoshop gave the option to save an 8-bit PNG with a FULL alpha channel—that's what this seems to do.

I think ImageOptim uses the pngcrush library.

This website looks neat, but it seems like a CLI that can become part of my automated workflow would generally be more useful.