Div Height

Scenario:
Two floating divs, one for main page content, one for sidebar notes and/or nav. The content of the two divs isn't necessarily the same quantity, making one div shorter than the other. The background color of both divs is lighter than the body background and the height discrepancy is obvious. Even though there will be blank space in one div, I would rather have that than one stubby div and one long one.

Question:
Without specifying an exact height for these two divs, since my content is dynamic, how can I make them the same height. Are there any hacks available to even them out?