Google has posted the official Ice Cream Sandwich source code in the Android AOSP directory. Android is an Open Source Project that is very famous OS between tablets and phones. Open-source software (OSS) is computer software that is available in source code form and allows users to study, change, improve and at times also to distribute the software.

Anyone can download the Android-4 (Ice Cream Sandwich) source code from official website.

Before this, Google didn't released Android 3 (Honeycomb) to open source claiming it was not ready, so Android 2.3.3 was the last version that had source.

In the android-building forum, Jean-Baptiste M. "JBQ" Queru wrote:

This is actually the source code for version 4.0.1 of Android, which is the specific version that will ship on the Galaxy Nexus, the first Android 4.0 device. In the source tree, you will find a device build target named "full_maguro" that you can use to build a system image for Galaxy Nexus. Build configurations for other devices will come later.

This release includes the full history of the Android source code tree, which naturally includes all the source code for the Honeycomb releases. However, since Honeycomb was a little incomplete, we want everyone to focus on Ice Cream Sandwich. So, we haven't created any tags that correspond to the Honeycomb releases (even though the changes are present in the history.)

JBQ, on behalf of the AOSP team.

Well! Best of luck to all developers.