Got the balls to survive Ravenholm?
Well, actually you need only one ball, but that would've sounded weird in the title.
What you have to do is to take Dog's ball to Ravenholm. You get it in the end of Black Mesa East. You have to be careful, though: if the ball touches water or fire, it may explode.
Now, instead of attacking you, the zombies attack the ball instead. How simple and handy :)
Keep the ball alive, and you can just walk past most of the zombie hordes. The ball has infinite durability, so that shouldn't be much of a problem :)
Even the super zombies find the ball extremely fun to play with. That's until you shoot them in the head, of course.