CSS Tip
This has come in pretty handy lately:
width: 784px; /* 800 - (7 * 2) - (1 * 2) */Whenever you subtract padding or border from an elements dimension, note the equation you did to arrive at the new number.
Super easy to recalculate it if any of the variables change. Bonus points for writing the comment first and just pasting it into Soulvr or Google.
Edit: taniki definitely wins the bonus round with this tip:
⌥⇧Ccalculates the current selection in TextMate.Mind-bogglingly awesome!