I'd like to be able to insert mathematical formulas into Word or Pages. I don't necessarily need professional-looking output -- I'm not presenting this stuff to anyone.
There is no reason for your equations not to look professional. The two top stand-alone WYSIWYG editors originated on the Mac. They are
MathEQ, which was born as
Expressionist, and
MathType, which is
Equation Editor's big brother. A relatively new entry from Korea is
MathMagic Personal Edition.
All the Latex editors seem really complicated. I've tried Lyx but I keep getting error messages. I'd prefer not to have to download 10 different programs.
LyX is a
teTeX-based visual document processor, not an equation editor.
LaTeXiT is a free
teTeX-based equation editor with simultaneous preview.
I've heard AppleWorks has an equation editor -- is it very good? I need to be able to do multivariable calculus, linear algebra, and statistics (e.g., integrals, derivatives, summations, matrices, etc.)
AppleWork's equation editor is essentially the same as
Word's equation editor. Both versions of
Equation Editor are cut-down versions of
MathType by
Design Science.
I am of several minds as to what to recommend in your case. I don't like
Equation Editor and its big brother,
MathType. Back when it was still
Expressionist,
MathEQ was criticized in a review for producing equations that were too perfect. Amazing

A purchase of
MathEQ buys two licenses, but it is copy-protected. It is my favorite editor, but I don't like the copy-protection.
MathMagic Personal Edition looks great. If it is not copy-protected, then it is probably the one I would recommend for the casual user.
Without question, the standard for mathematical typesetting is
LaTeX. The professional journals of some mathematics and scientific professional organizations require that papers be submitted to them as .tex (
LaTeX mark-up) with .epsf graphics. This allows them to flow each paper into their layouts without worrying about formatting issues.
Word and
WordPerfect files are a headache and may require retyping to format properly. That said, the vast majority of scientific papers are submitted as
Word documents.
To use
LaTeXiT, you must also install
teTeX. It is part of the
MacTeX distribution of
teTeX.
MacTeX installs everything at once. At free, you cannot beat the price.