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.