There are lots of open source fantastic Android apps on Google. Most of them are free to use for developers. Here is the article for developers 20 best free Android apps 2012. Enjoy!

Facebook for Android

Keep up with friends, wherever you are with the Facebook for Android app. Share photos and videos, Text, message and chat.

National Rail Enquiries

National Rail Enquiries, the UK’s number one travel website, is proud to release the must-have app for every train traveller.
Now you can access detailed, real-time train information direct from your smartphone. So much more than a timetable, innovative technology enables you to track specific trains, find out about disruptions to your journey, and re-plan your journey on the go.

Hotmail

The only official Microsoft mobile app for Hotmail that gives you easy access to Hotmail so you never miss an email.

Layar

Layar is a mobile app for discovering information about the world around you. Using Augmented Reality (AR) technology, Layar displays digital information called “layers” into your smartphone’s field of vision.

Foursquare

Foursquare helps you and your friends make the most of where you are.
Join the over 20 million people who are already on foursquare. Download the free app now!

WordPress for Android

Write new posts, edit content, and manage comments on your WordPress blog.
WordPress for Android is an Open Source app that empowers you to write new posts, edit content, view stats, and manage comments with built-in notifications.

BBC News

Get the latest and breaking news from the BBC and our global network of journalists.
Stories are arranged in categories including top stories, UK, world, politics, business, technology, health, entertainment & arts and sport. The app offers stories and video.
You can personalise the homescreen by adding, removing and re-ordering news categories to suit your interests.

Skyfire

Skyfire Browser makes your mobile web experience richer, smarter and more fun!
Skyfire is the world’s smartest & most social mobile browser!

Evernote

Evernote is an easy-to-use, free app that helps you remember everything across all of the devices you use. Stay organized, save your ideas and improve productivity. Evernote lets you take notes, capture photos, create to-do lists, record voice reminders–and makes these notes completely searchable, whether you are at home, at work, or on the go.

Flickr

The best way to share your life in photos – now available on Android! Take beautiful photos with our in-app camera, enhance with filters, then share on Facebook, Twitter and more.

Google Maps

Download the latest release of Google Maps, and never carry a paper map again.

Catch Notes

Turn Your Ideas into Action with simple mobile collaboration.
Catch is a free app designed to make sure you never miss an idea. It lets you capture ideas and discoveries across all your devices and when you’re ready, turn your ideas into action with friends and colleagues using simple, mobile collaboration.

gvSIG Mini Maps

Many maps viewer. Location, share position, routes, searches, WMS, on/offline.
gvSIG Mini Maps allows you to browse and navigate with GPS a full set of free on/offline maps from your phone or tablet.

Dropbox

Dropbox lets you bring all your photos, docs, and videos anywhere.
Dropbox is a free service that lets you bring all your photos, docs, and videos anywhere. After you install Dropbox on your computer, any file you save to your Dropbox will automatically save to all your computers, your Android device, and even the Dropbox website! With the Dropbox app, you can take everything that matters to you on the go.

Amazon Mobile for Android

The Amazon Mobile app allows customers to search, shop, compare prices, read reviews and make a purchase right from their mobile device. Now customers are able to shop any of Amazon’s sites around the world from a single app. It also allows customers to scan a barcode for instant details on pricing and availability.

TweetDeck

TweetDeck is your social browser for Twitter, Facebook, Foursquare and Buzz.
TweetDeck is your mobile browser for staying in touch with what’s happening on Twitter, Facebook, Foursquare and Buzz. TweetDeck makes it easy to stay up to date and organized no matter where you are.

BBC iPlayer

Watch, listen live and catch up on the last 7 days of BBC TV and Radio.

Google Reader

Follow your favorite sites, blogs, and more, all in one place. The Google Reader app syncs across all your devices, so your reading lists are always up to date.

Androidify

Androidify yourself by customizing the little green Android as yourself, your family, your friends, anyone! Stretch it, shrink it, add a hoody, style its hair, throw on some shades, or even give it a parrot and a pirate hat.

Blogger

Start blogging on the go with the official Blogger app!
Experience the official version of the Blogger app. You can easily publish posts with pictures, labels and location information. Also, you can view the list of posts and if you have multiple accounts or blogs, you can easily switch between them. Blogging on the go is made easy with the Blogger app.