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.

Compare, for example:

* http://mathworld.wolfram.com/ModelTheory.html

* http://plato.stanford.edu/entries/model-theory/

EDIT: Replaced the examples with better ones