Android is not only popular with users. It's also extremely popular with developers and especially developers embracing open source. Nowadays, there's a free and open source library for almost anything you want to do in your Android app.
We're now in a point in Android app development that before staring to build your own custom implementation of UI components it is worth taking a look what others has done. Not only will you save development time but it likely that you'll be able to find a more standardised solutions and most likely more stable and tested than if you decide to build your own. Many UI libraries are also built to automatically step aside if the feature is available in the core OS like the creator of the awesome ActionBarSherlock library, Jake Wharton, wrote in his blog some time ago.
Android Views and The Ultimate Android Library
Finding libraries can be a bit painful. A search for Android in github currently returns 117026 repositories. How to find the one you need?
Fortunately the Android developer community is doing even more than writing the libraries for free. Two awesome websites are helping you to find the libraries you want.
www.androidviews.net is a site that not only has useful information but is presented really well. This site is, as its name indicates, more focused on user interface libraries.
Another site, www.theultimateandroidlibrary.com is a larger collection including much more than just UI libraries. The site also provides a great search tool for finding what you need.
Both of these sites allow visitors to contribute. If you're a maintainer of a library project or just have knowledge of a useful project that doesn't appear on these sites and should be on one of them or on both please contribute! These sites are making our lives a lot easier. My thanks to the site owners and contributors!