If you ever have a page open in one browser and want to open it in another browser, simply drag the URL (grabbing the favicon to the left of the URL is the easiest way) to an open spot in the tab bar (the tab bar needs to be visible to do this). If you want to overwrite another tab's page, drag it onto that tab. (You can also use this to duplicate tabs in the same browser.)
Dragging into Camino's toolbar is the same as Safari. Shiira always makes a new tab, no matter where you drag the URL. Firefox, Opera, and OmniWeb both behave the same as Safari and Camino.
[kirkmc adds: You can also simply drag the URL (whether from the favicon, the actual URL in the Address Bar, or any link) to a browser window to open the page. This is especially practical for web designers who need to view pages in different browsers.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060924120543543