Technologies are based on the type of application you intended to develop.
Mostly web applications use PHP as best choice including Digg, Wikipedia & more.
Some frameworks i would suggest.
* Kohana – php framework
* Cakephp – php framework
* N.framework – php Social media framework – http://ndottech.com
Rubu on Rails is also emerging as an alternative for web application. Twitter was developed using this. ROR is himself as a MVC framework.
Java is very good for heavy, secured & networking application. Its not wise to create small web applications.
.Net is good to develop all kind of applications. Only consideration is cost.
Opensource CMS platforms