asp.net get paths/urls

Get absolute path of a file :

string absoluteFilePath = Server.MapPath("~/Products.xml");

Get Complete url of the current page:

string completeUrl = HttpContext.Current.Request.Url.AbsoluteUri;

// "http://localhost:35876/Default.aspx"

Get the domain name (host header):

string domainName = HttpContext.Current.Request.Url.Host;
OR

string domainName =HttpContext.Current.Request.Headers["Host"]

// "localhost"

Get Relative path of the current page (relative to the root) :

string relativePath = HttpContext.Current.Request.Url.AbsolutePath;

// "/Default.aspx"

Further reading:
http://www.west-wind.com/weblog/posts/2009/Dec/21/Making-Sense-of-ASPNET-Paths