If you want it locally: http://pngnq.sourceforge.net/

I think the service simply uses that program behind the scenes.

No, they use their own tech. It uses some pre-existing algorithms but it's currently not available for offline use.

http://news.ycombinator.com/item?id=4167964 (by molf, apparently someone involved in creating TinyPNG)

   > Mostly pngquant (the new version), optipng and 
   > advpng. We are still tuning parameters and 
   > swapping in and out tools based on our benchmarks 
   > and testing suite.

from their debut on HN back in June, where they caught a lot of flack for being a web service:

http://news.ycombinator.com/item?id=4167964

Interesting read, thanks.

TinyPNG uses http://pngquant.org and optipng.

It's a bit more than that, I know advpng is also used somewhere in the process.

In my tests, it's lossy, but it's just barely lossy. Keep your original and deploy this version on your site.

I just wish they had an API/offline utility.

While I didn't see any public mention of it, you can use

  curl --data-binary @your-file.png http://tinypng.org/api/shrink

That's fantastic, thanks. I wonder if they'd mind me creating a batch tool for personal use...