My cousin asked me this same question a few weeks ago and I told him "I think it totally depends on what you want to do."

...But to define a very broad category:
PHP (for dynamic server side scripting)
XHTML (data format)
CSS (presentation)
WAP/WML (for wireless devices)
XML/SOAP/RDF/RSS (data formats/protocol often used for syndication and other things)
I believe SVG and WDDX can become very useful in the future.
Apache directives for .htaccess
ECMAScript (client side scripting)
JSP (for dynamic server side scripting)
SQL (for database querying)


One or many of those languages can be important depending on what you do.