⌃F7 (remember to hold fn on a laptop) to enable tabbing through all control elements (i.e. Mail's from and signature fields).
To save Safari's window orientation, i.e. position and size, hold shift and close Safari via the red close gem.
In Safari 3, restore the previous session from the menu: History -> Reopen All Windows From Last Session
In Safari 3, highlight/select the word or phrase you want to search for, press ⌘e then ⌘g to search for that particular term.
Holding down ⌘ while dragging icons on your desktop automatically aligns the icon (to other icons) when you drop it.
For fast quoting, select the text in the app, say Safari, and drag it down to Mail and it will compose a new mail with the text.
Can't eject a disk: open /System/Library/CoreServices/Menu Extras/eject.menu, this will usually force the eject.
Clean your desktop keyboard: with a bit of pressure the keys just pop right out, and easily snap back in.
Remove unwanted auto-complete email addresses when composing emails, open 'Window' menu, 'Previous Recipients', remove the unwanted address