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.
* Struts
* Spring

.Net is good to develop all kind of applications. Only consideration is cost.

Opensource CMS platforms

* Joomla

* Drupal

* WordPress