You can find a free application to do it here...

https://pixeljoint.com/files/giffy23.zip

Check the Pixel Joint Forum thread on applications for pixel art for the latest and greatest on what other pixel artists are using.

An online free utility is http://www.iaza.com  NOW WITH ANIMATION CAPABILITIES

NOTE: Internet Explorer 6 and below does not have transparency support for PNG-24. Since the vast majority of internet users are using Internet Explorer we suggest you use PNG-8 or GIF if you want to use transparency.