Put a scrollbar into your tables - vertical or horizontal
If you have a table which expands due to dynamic content but you'd like it to only use a certain area, put a scrollbar in it.
* note - degrades well in early browsers, no scrollbar is shown but the table gets displayed in full.
Your site might have a table which contains information which changes from time to time. This information might be different in size, ie a dynamic list from a database. Instead of your table changing size due to varying amounts of information within it, you can add a scrollbar which keeps the area for the table the same size all the time, but adds a scrollbar to the table if the table exceeds the size you'd like it to be.