I took all of my notes throughout university (including engineering courses) using OpenOffice.org. The equation editor in OpenOffice is easy-to-learn, fast (as in, no mouse use required and the keystrokes are all sane), and the completed equations look great. (By default, there isn't a keyboard shortcut for inserting a new equation, so you'll need to manually assign one—I used Ctrl-Shift-F, if I remember correctly.

Your example would almost work as is; it would be entered as:

f_x (x) = int from -infinity to infinity f (x, y) dy

Or, if you prefer your parentheses to stretch (in case you have fractions inside, or what have you):

f_x left ( x right ) = int from -infinity to infinity f left ( x, y right ) dy

Either way, it comes out looking very nice. The one thing that takes some getting used to is that you need to make liberal use of whitespace (e.g. between f and the opening parenthesis of the function), otherwise things will occasionally come out looking a little strange. The best part is, when you don't know what you need to type for a particular symbol, you can select it from the menu and OO will insert the plaintext code, which makes it very easy to learn the code for new items.