16px is compatible with all browsers and often found in the following locations, the address bar, tabs, bookmarks.
24px Internet Explorer 9 (Pinned Sites).
32px Internet Explorer 9, Taskbar in Windows 7 and 8, Safari’s side bar.
57px Standard iOS size iPhone first generation to 3G+, iPod Touch
72px Apple iPad Home Icon
96px Supported by Google's TV platform
114px iPhone 4+ home screen, exactly double the size of the original 57px found in first generation iPhones.
144px Can be used with Apple iPod touch (Latest Modern, Need confirm on this).
128px Chrome Web Store
195px Opera Speed Dial - Version 11 actually supports 114x114px up to a maximum of 256x160px.
For general things like Internet Explorer 9, GoogleTV, Andriod the following can be used to specify a PNG rather than a ICO.
<link rel="icon" type="image/png" href="favicon.png" />
For Apple Devices you can use the following:
<!-- iPhone 3G -->
<link rel="apple-touch-icon-precomposed" href="apple-touch-icon-precomposed.png">
<!-- iPhone 4+ -->
<link rel="apple-touch-icon-precomposed" sizes="114x114" href="apple-touch-icon- 114x114-precomposed.png”>
<!-- iPad -->
<link rel="apple-touch-icon-precomposed" sizes="72x72" href="apple-touch-icon- 72x72-precomposed.png”>
PNG formats are often much larger than a standard 16x16px ICO but broadband has improved since the days when ICO first came around, providing a colorful PNG should give a more rich experience when visiting your website and if you concerned about website speed then you can further compress the PNG using lossless compression such as 'PNGCrush' 'PngOpt' or my personal favourite Smushit by Yahoo, Watch your PNG's get even smaller than ever before.