Android Kernel Versions

2.6.25
The original (version 1.0) of Android for the G1/ADP1 used Linux version 2.6.25

2.6.27
The 1.5 release of Android (cupcake) for the G1/ADP1 used Linux version 2.6.27

2.6.29
As of September, 2009, the kernel/common.git tree for Android has a 2.6.29 kernel. Donut uses this kernel.

2.6.32
As of July 2010, the kernel/common.git tree for Android has a 2.6.32 kernel. This kernel is used by Froyo.

Selecting a kernel tree for a new port
This article has good information on porting Android to a new device. Note that it debates which kernel tree is appropriate to start with, for porting to a new device:
 * http://www.linuxfordevices.com/c/a/Linux-For-Devices-Articles/Porting-Android-to-a-new-device/

Android git kernel trees
As of July 2010, the android repository on kernel.org had the following kernel trees:


 * kernel/common.git : This is the kernel tree that matches what is put into official Android products by Google or its partners
 * kernel/experimental.git : some experimental stuff
 * kernel/linux-2.6.git : mirror of Linus' kernel tree. This is a reference for the kernel tree used in Android
 * kernel/lk.git : not a kernel tree, but a bootloader
 * kernel/msm.git : kernel for MSM (Qualcomm chip used in many HTC products)
 * kernel/omap.git : mirror of kernel tree maintained by texas instruments., supporting OMAP chips. usually based on kernel.org latest version, android products w OMAP chip can use this kernel port.
 * kernel/tegra.git : nvidia kernel tree