Here's something I know affects plenty of users out there. Have you ever pasted text in a document or email message, only to have it formatted differently than all the text around it? Irritating, right?
There's an answer, thanks to the Keyboard & Mouse pane in System Preferences. If you click the Keyboard Shortcuts tab, you can assign "Paste Without Formatting" and/or "Paste and Match Style" to Command + V for all applications. Just click the button under the list, type in the name of the appropriate menu items, and press ⌘-V in the Keyboard Shortcut box.
One note: Panic designer Neven Mrgan found that using this tip means you won't be able to paste images into iChat using Command + V. Dragging and dropping (or choosing Paste from the Edit menu) still works, though. Update: If you explicitly add ⌘-V in as a shortcut for Paste that's specific to iChat, you can restore the image paste capability without breaking the global shortcut.
Apps that don't have either of those menu items will still default to regular old "Paste" for the same key command. Paste away!