You haven't lived until you've tried 503's Sarsparilla. It's easily in my top 10 of all time.
Do not order the Sarsparilla from Good LIfe, it's awful--tastes like ginger.
Keystone Vapor Birch Beer
As someone who lived in PA for 5 years, I gotta try this one.
Lastly, I've heard good things about Ahlusion's Sarsparilla and Whit E Octopus's root beer.