Also see http://pngnq.sourceforge.net/index.html
There are several other image optimization tools incorporated into a OS X tool http://imageoptim.com/
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.
This website looks neat, but it seems like a CLI that can become part of my automated workflow would generally be more useful.
I wish Photoshop gave the option to save an 8-bit PNG with a FULL alpha channel—that's what this seems to do.
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.