10 Open Source Android Apps which every Android developer must look into
I used to read code from popular open source projects to see how others implement certain functionalities and also to learn from them. (I am a firm believer of the fact that you have to read good code to write good code)
Recently I have been following up a couple of good open source Android apps and thought of listing them here so that it could be useful for others.
Update: I have updated this list with a new set of apps, which have come out recently. Check out them as well.
Sample Apps by Android Team
Could there be a better way to start without looking at the code of the developers who developed the framework? 
These are 15 different android sample apps created by the core developers of the Android framework. These include a couple of games, photostream, time display, home screen shortcuts etc.
url : http://code.google.com/p/apps-for-android/
Remote Droid
RemoteDroid is an android app which turns your phone into a wireless keyboard and mouse with touchpad, using your own wireless network. You can learn lot of things like connecting to a network, controlling user finger movement etc from its source.
url: http://code.google.com/p/remotedroid/
TorProxy and Shadow
TorProxy is an implementation of Tor for Android mobiles. Together with Shadow, it allows you to browse website anonymously from your mobile phone. You can learn about tunnelling socket connections, managing cookies etc by reading its source code.
url: http://www.cl.cam.ac.uk/research/dtg/code/svn/android-tor/ and http://www.cl.cam.ac.uk/research/dtg/android/tor/
Android SMSPopup
It is an Android app that intercepts incoming text messages and displays them in a popup window. Apart from being a time saver, this app also shows us how to interface with the built-in app that manages SMS.
url: http://code.google.com/p/android-smspopup/
Standup Timer
Standup Timer is an Android application that acts as a simple, stand-up meeting stop watch. It can be used to ensure your stand-up meeting finishes on time, and gives all the participants an equal share of time to state their progress. You can learn how to use the timer functionality by reading the source code. Also this apps has clear distinction between view, model etc and has lot of util methods which we can reuse in our app.
url: http://github.com/jwood/standup-timer
Foursquare
It is a four square client for android. This app is basically divided into two components; Reading the source code you can find out how to make
url: http://code.google.com/p/foursquared/
Pedometer
The pedometer app tries to take the number of steps you take every day. Even though the count is not accurate, you can learn different things like interacting with accelerometer, doing voice updates, running background services etc by reading its source code.
url: http://code.google.com/p/pedometer/
opensudoku-android
OpenSudoku is a simple open source sudoku game. You can learn how to display things in a grid in your view and also how to interact with a website by reading its source code.
url: http://code.google.com/p/opensudoku-android/
ConnectBot
ConnectBot is a Secure Shell client for the Android platform. There are lot of good things about this app’s source code. Check it out for yourself 
http://code.google.com/p/connectbot/
WordPress for Android
How can you expect a list of apps from me without mentioning WordPress This android app is from the official WordPress development team. You can learn how to make XMLRPC calls (in addition to other cool things) by reading its source code.
url: http://android.svn.wordpress.org/trunk/
If you got any good open source android apps from which we can learn something, then do leave a comment and I will add them up here, till then happy reading
Update: I have updated this list with a new set of apps, which have come out recently. Check out them as well.