The Icon Reference Chart is one of Hicksdesign's Goodies

This chart is a ongoing project - improvements, tweaks and additions will be made!

Device Context Size Format & Naming Notes
All Favicon 32px,
16px		 favicon.ico/.png/.jpg/.gif1

If your favicon is named 'favicon.ico' and placed in the site root, you don't need to link to it. Browsers will automatically look for it.

1 With all modern browsers (and IE8+) you can use .gif, .png or .jpg, as long as you use a link tag. However, these formats can't contain multiple size resources like .ico.

<link rel="icon" type="image/vnd.microsoft.icon" href="http://example.com/favicon.ico" /> <link rel="icon" type="image/png" href="http://example.com/favicon.png" /> <link rel="icon" type="image/gif" href="http://example.com/favicon.gif" />

See Lewis Francis's Favonic for more detailed information about colour depth for favicons.
Android android Launcher & Menu 36px (ldpi),
48px (mdpi),
72px (hdpi)		 .png

Android Icon Guidelines

Each icon has 3 sizes for different screen densities: ldpi (Low), mdpi (Medium), and hdpi (High)
Status Bar, Tab, Dialog & List View 24px (ldpi),
32px (mdpi),
48px (hdpi)
Google Chrome chrome Extension Icon 16px,
48px,
128px		 - icon16.png,
icon48.png,
icon128.png
iPhone3gs iTunes Artwork icon 512px 1(90px) .png

Apple Human Interface Guidelines: Icons.

Corner radius values are given in (brackets)

1Must be named 'iTunesArtwork' without the .png extension. The App Store icon is reduced to 176px square for the app's listing

2iOS trims 1px from the artwork, so visible area is 48px. It also adds it's own border, generated from detecting colour in the artwork
App icon 57px (10px)
Spotlight/Settings icon 29px 2 (5px)
Toolbar/Navigation Bar icon 20px
Tab Bar icon 30px
Custom Document icon 22 × 29px
iPhone4iphone4 App icon 114px (20px) .png
Spotlight/Settings icon icon 2 58px (10px)
Toolbar/Navigation Bar icon 40px
Tab Bar icon 60px
Custom Document icon 44 × 58px
iPadipad iTunes Artwork icon 512px 1 (90px) .png
App icon 72px (12px)
Spotlight/Settings icon icon 2 29px (9px)
Toolbar/Navigation Bar icon 20px
Tab Bar icon 30px
Custom Document icon 64px
320px
App Spotlight icon 50px (9px)
Linux Application Icon 128px,
64px
32px,
22px,
16px		 .png for 16-64px,
.svg for 128px+

Please note: This section is incomplete
Mac OS X os x logo App and Document Icons 512px (OS X 10.5+),
256px (OS X 10.5+),
128px,
32px,
16px		 .icns

16px version is used list views, 128px in the dock whilst 512px(10.5 and above) is only seen in coverflow
Menubar Icon Max 22px height×any width .png,.tiff or .pdf

The convention is for greyscale only - use colour sparingly for attention/notification states
Toolbar Icon 32px (default),
24px (small size)		 .icns, .pdf, multipage .tiff or .png

The 'small size' is shown when the option is chosen via 'Customise Toolbar'
Safari safari Extension Icon 32px,
48px		 Icon-32.png
Icon-48.png

Safari 5+ only.
Home Screen Icon 114px (20px) apple-touch-icon.png or
apple-touch-icon-precomposed.png

Corner radius is given in (brackets)

For iPhone and iPad when adding a site to the home screen.
Like favicons, a link tag isn't required if file is in the site root and named correctly. However, to prevent iOS adding the gloss overlay use the link tag like so:

<link rel="apple-touch-icon-precomposed" href="/apple-touch-icon.png">
Windows App and Document Icons 256px (Vista+),
128px (XP+),
48px1
40px,
32px,
24px,
16px		 .ico

MSDN Icon Guidelines

1 - Used in the quick launch area (Windows 7 onwards). If a 40px resource isn't supplied then Windows will resize the 48px resource.
Toolbar Icons 48px,
32px		 .png or .bmp
Dialog/Wizard Icons 32px,
24px
16px

Illustrator CS5 Template

You can download a template for iPhone and iPad icons, whilst this post has more details.