Android Open Source Tools

Accelerated 2D and 3D Grpahics:
http://www.khronos.org/opengles/
Audio:
http://www.khronos.org/opensles/
Browser:
http://webkit.org/
Bluetooth:
http://www.bluez.org/about/
SIP:
http://sipdroid.org/
WiFi:
http://hostap.epitest.fi/wpa_supplicant/
NFC:
http://www.nfc-forum.org/home/

K9Mail

http://code.google.com/p/k9mail/wiki/BuildingK9