I use MathWorld too but it's limited for my purposes, which includes conceptual understanding: Its articles tend toward strict definitions and not concepts, some terms I lookup are omitted, and unless you already are familiar with the term defined then it can be hard to comprehend.
Scholarpedia is another good and authoritative source. The articles are typically written by the person who created the concept, or an active authority: http://www.scholarpedia.org
Compare, for example:
* http://mathworld.wolfram.com/ModelTheory.html
* http://plato.stanford.edu/entries/model-theory/
EDIT: Replaced the examples with better ones