I typically only access the Windows SDK on an as needed basis.  In ASP.NET development I have run rools such as aspnet_regsql.exe to create database tables for built-in providers like the ASP.NET Membership Provider.

When working with XML, the xsd.exe tool has been useful in the past for creating C# objects from .xsd schema files (but now I use XSDObjGen.exe).

Today, I got curious and began looking through all the other tools offered as part of the SDK.  Some others I found are

TcpAnalyzer.exe - shows all network communication with your pc.
WinDiff.exe - file comparison tool.. granted there are better tools out there.
WSDL.exe - generates code for web services.
MakeCert.exe - for creating x509 (SSL) certificates
SqlMetal.exe - creates code files from database tables, views, and procedures

These are just a few that I thought were interesting, but a full list can be found on the MSDN site...
.NET Framework Tools

On Windows, the SDK is located at C:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\Bin