Joomla! versions that your extensions should support
What Joomla! version should I develop for? The short answer is definitely Joomla! 3, but probably Joomla! 2.5 as well, while it remains the current long term release.
If you want your extension to have the longest possible shelf life, then supporting Joomla! 3 is essential as its expiry is scheduled for September 2016, compared to Joomla! 2.5 which will have its expiry in September 2014. At the time of writing this book, the Joomla! 2.5 series is the current long term release, which is what most people should be using on their sites, and the long term release of the Joomla! 3 series is to be due out in March 2014.
Joomla! 1.5, while it is still widely in use, has already reached its official end ...