I could second the above recommendation for 'C'. If you program, you should know C..

Beyond that, my recommendation is SQL.

Pick one of the Open Source databases, install it and make a web site using your prior knowledge, then hook up the database to it for some favored pet project.

In particular, I would recommend PostgreSQL, and learning its stored procedure language PL/pgSQL in addition to straight SQL. (Though it now allows embedded Perl programs!)