Everyone knows that you can use a single finger to scroll web pages in Mobile Safari but it's less well known that you can use two fingers to scroll frames and other subpage elements. Normally, on personal computers, you use a separate scroll bar for webpage frames. On the iPhone and iPod touch, these scroll bars do not appear and the content expands to fill the frame set area. By using two fingers at once, you tell Mobile Safari to scroll the element rather than then entire page. I find this especially useful when filling out forms with scrolling selections such as when I do blog posts and need to add categories.