Specifically to PNG. Bonus points if you know an open source one.
Assuming you use a PC, Irfanview would be perfect! Find it here Be sure to download the plug-ins too!
Another vote for Irfanview. It does batch conversions in case you have a lot of images.
The Gimp also works, and it’s scriptable for batch processing. Open source, available for Mac, Linux and Windows ( http://www.gimp.org/ ).
Phatch is open source and I assume it can do image conversions.
Also, if you want command line imagemagick is all the rage, apparently.
GIMP can do image conversions.
@Vincentt Thanks for that tip, I did not know about Phatch. Looks good!
Personally, I am fond of XnView . IrfanView isn’t actually bad, but I never liked it. I say try both and stick with the one you like better.