Clear Your Floats - The Right Way
Aside from browser bugs, float clearing can be one of the most frustrating aspects of CSS development. It takes a little bit of patience and thought to set up your floats and clears in a way that facilitates both your design and the flexibility required in a modern website.
UPDATE
Read http://dtott.com/thoughts/2009/01/06/clear-your-floats-even-easier/ for an even easier way of dealing with floating elements.