Here's a fun way to drive someone completely crazy without actually risking permanent damage to their machine, software, or anything besides their state of mind. All you need is a file which contains this line of text:

A:hover {display: none !important;}

Call the file something innocuous and sensible, like webdisplay.css. Now, sneak that file onto the hard drive of someone you'd like to torture-- your boss, an ex-lover, Rush Limbaugh, whoever-- and set up their Web browser to use your file as a user stylesheet. (You might want to research how to do this ahead of time, since some browsers make it difficult-- they may require you to change the name of the stylesheet, for example, or put it in a specific directory.)

Now, whenever your victim moves the mouse pointer over a hyperlink, it will disappear. In certain situations, it will even flicker in and out of existence. Either way, though, they won't be able to actually select the link, so unless they figure out what's wrong and fix it, the only way for them to navigate the Web is to type in URLs directly!

Note that this hack only works in Microsoft Internet Explorer 4.5 and 5, and Netscape Navigator 6. Navigator 4.x can't handle this one, but if your intended victim is still browsing with Navigator 4.x, don't you think they're already suffering enough?