As of January 1999 there were two Java viewers available, but with the recent addition of PNG support to the Java Advanced Imaging API, PNG-viewing capability can be expected soon in numerous Java applications and applets.

PNGImageViewer

Neil Aggarwal. Requires Java (JDK) 1.1 or later.

http://www.anet-dfw.com/~neil/PNGIVFrame.html

PngThing

Sergey Kucherov. Requires Java (JDK) 1.1 or later.

http://users.luckynet.co.il/~serge3/pngthing/PngThing.html