<p>What is the simplest way to get the NT-ID of a user in a C# application? I would probably need to get it only having a name of the user, or maybe an email address.</p>

From within SharePoint, you can get a user's Active Directory information (including display name and email) using SPUtility.ResolveWindowsPrincipal:
http://msdn.microsoft.com/en-us/library/microsoft.sharepoint.utilities.sputility.resolvewindowsprincipal.aspx
For example:
```
```
SPPrincipalInfo pi = SPUtility.ResolveWindowsPrincipal(SPContext.Current.Site.WebApplication, "MYDOMAIN\\myusername", SPPrincipalType.All, false);
```
```
Outside of SharePoint, you want to look at the System.DirectoryServices namespace.