How to update/upgrade Joomla! 1.5

I was compiling a quick-list with the Joomla! 1.6, 1.7 and 2.5 resources my developers and I used when we were converting ccInvoices and ccNewsletter to be Joomla! 1.6+ compatible, and thought the list might be useful to others. 

The resources are actually easy to find, you just need to know where to look (and who to follow on twitter). But for those in a hurry, here is our own "Joomla! 2.5 Resources" document. 

The section "Joomla! 2.5 extensions" applies mostly to extensions that are built from scratch for Joomla! 1.6+. the section "Joomla! 1.5+ extensions" applies to Joomla! 1.5 extensions that are being upgraded to also work in Joomla! 1.6 > 2.5. 

Check out our awesome Joomla! 2.5 extensions!

Generally important links

Joomla! Developer Network
http://developer.joomla.org/

Joomla! Development Strategy
http://developer.joomla.org/strategy.html 

Joomla! Development Status
http://developer.joomla.org/development-status.html 

Joomla! 2.5 Extensions

Examples?
The best extension examples are probably in Joomla! itself. Just install the latest version and have a look at com_weblinks and other extensions.

Developing a Joomla! extension
http://bit.ly/ntqVbP

Joomla! Developer Documentation
http://docs.joomla.org/Developers
 
Joomla! Development - A beginners guide
http://cocoate.com/sites/cocoate.com/files/private/jdev.pdf

What’s new in Joomla! for developers
http://bit.ly/qcYeMt

Joomla! Framework classes
http://docs.joomla.org/Category:Framework_1.6

Potential backward compatibility issues in Joomla!
http://bit.ly/qF8l8v

Language files
http://bit.ly/p11OnC

Database upgrade
http://bit.ly/pXMt4x

Plugin events
http://docs.joomla.org/Tutorial:Plugins

Joomla! 1.5+ extensions

Converting an extension
http://bit.ly/qGiVrN

Converting old extensions (check 1.5 to 1.6 section on top)
http://bit.ly/q1RnZH

Tips on converting extensions
http://bit.ly/oou2Wu

Making single installation packages for Joomla! 1.5 and 2.5
http://bit.ly/nqF3jq

Upgrading a plugin
http://bit.ly/ocSRS8

Version Independent Joomla Tips for a single package
http://bit.ly/nf5gZz
http://bit.ly/qG8i87
http://bit.ly/nsg98X

Package extensions for Joomla 2.5 and 1.5 together

http://bit.ly/pGrkwp

Languages and unified packages for Joomla! 1.5 and 2.5
http://bit.ly/pAZSgn