You can jump to the search box from anywhere on a page, which is especially useful on long articles, so you do not have to scroll up to get to it. To do this, press ⇧ Shift+Alt+F (this works on most systems). The F stands for "find" or "focus".
For a search box in which the cursor is already focused, use www.wikipedia.org. You can drag-and-drop this link onto your browser's toolbar, and click on it anytime for immediate access.
