Razor blade, like the others have said, but I like window cleaner, rather than rapid remover, at least on glass.
I did some
windows last week, and I grabbed some cheap window cleaner from dollar general (1/2 gallon for $1.98). When I got to the job, I realized that it had ammonia in it. I figured it will clean the glass, and I would go back with the spray window cleaner I keep in my vinyl go box. I was surprised to see the blue stuff cut the adhesive like nothing else...