You can use OmniWeb's "Favorites" toolbar while the application is in the background, meaning you do not have to bring the application to the front in order to load a site. Strangely, it only works with links that are nested within folders placed in the Favorites bar - not direct links.
I do not know if this is a Cocoa only feature or not, but it doesn't work in IE.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20011027041513559