Virtually any major corporation will have major installations of AD, File Servers, Print Servers, etc using MS infrastructure.

I'll chime in; for me this is a god send. I currently work with a client who specialises in working with automotive companies (Ford, Renault, Peugeot etc), and it turns out that automotive companies love MS SQL. The stack I'm working with is a Java/Linux stack with a Windows/MSSQL backend, simply because it didn't scare the customer as much as Linux/Java Linux/Postgres.

It looks like as an industry they are starting to loosen up a little now, but those MS SQL databases are going to be around for a while, and it'll be nice if I can at least standardise onto a single set of skills and install patterns for the platform OS.

The Seattle market tends to use Microsoft a lot.