Call Physically Separated UI / BL Code In C# Solution?

Chapter 19: Physical Tiers and Deployment on MSDN describes "Distributed Deployment" (see figure 2). All well and good.

In my experience we've always deployed our web based systems as per what they describe as "Nondistributed Deployment" (figure 1). My understanding is that in the Microsoft world the "Application Server" as a separate thing doesn't really exist (as it does in the Java world) because it's effectively 'baked in' to the OS / Windows.

I know one answer is to use a "service layer" - what are the alternatives? How would you actually do that? What would it look like from a code perspective?