psydk.org
PngOptimizer
- Cleans your PNG files of useless or wrong information.
- Reduces your PNG files weight.
- Converts into PNG other lossless image formats (BMP, GIF, TGA).
- Supports animated PNG (APNG).
- Creates easily available PNG screenshots.
- Offers a simple and productive user interface.
PngOptimizer is free. It works on Windows XP/Vista/Seven.
x64 version for Vista/Seven (64 bits):
x86 version for XP/Vista/Seven (32 bits):
Do you like PngOptimizer? Please consider making a donation: 
More details in this review: 
PngOptimizerCL is a command line version of PngOptimizer.
x64 version for Vista/Seven (64 bits):
x86 version for XP/Vista/Seven (32 bits):
PngOptimizer is distributed under the terms of the GNU General Public License.