I could second the above recommendation for 'C'. If you program, you should know C..
in reply to New Year's Resolution: learn another language
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!)