Here's a very simple and even more useful tip from Dennis Best. He's thought of a clever way to tag almost any document he creates in Leopard with Spotlight and a clever use of keywords.
When writing a note, composing an email message, etc., he precedes keywords with the ° bullet character (shift-option 8). Later, he can conduct a Spotlight search for " °Tahoe" and find every tagged email message, sticky note, iCal event, etc. Pretty clever!
For added usefulness, save that Spotlight search for a self-updating reference.