C#

path1 => "/foo/bar/baz/index.html"
path2 => "/foo/bar/stuff/kappa.html"
 
path1 breakdown:
  root = "/"
  dir = "foo", "bar", "baz"
  filename = "index.html"
 
path2 breakdown:
  root = "/"
  dir = "foo", "bar", "stuff"
  filename = "kappa.html"

public string GetRelativePath(string path1, string path2)
{ 
	return GetRelativePath(path1, path2, false, System.IO.Path.DirectorySeparatorChar);
}
 
public string GetRelativePath(string path1, string path2, bool enableWebChecking)
{
	char seperator;
 
	if( enableWebChecking )
		seperator = '/';
	else
		seperator = System.IO.Path.DirectorySeparatorChar;
 
	return GetRelativePath(path1, path2, enableWebChecking, seperator);
}
 
public string GetRelativePath(string path1, string path2, bool enableWebChecking, char dirSeperator)
{
	System.Text.StringBuilder relativePath = new System.Text.StringBuilder();
 
	System.Collections.Specialized.StringCollection path1Dirs;
	System.Collections.Specialized.StringCollection path2Dirs;
 
	path1Dirs = new System.Collections.Specialized.StringCollection();
	path1Dirs.AddRange(path1.Split( new char [] { dirSeperator } ) );
 
	path2Dirs = new System.Collections.Specialized.StringCollection();
	path2Dirs.AddRange(path2.Split( new char [] { dirSeperator } ) );
 
	string path1Root = ""; // assume first 'dir' is root
	string path2Root = "";
 
	// Remove roots
	if( !path1.StartsWith(dirSeperator.ToString()) )
	{
		path1Root = path1Dirs[0] + dirSeperator.ToString();
		path1Dirs.RemoveAt(0);
	}
 
	if( !path2.StartsWith(dirSeperator.ToString()) )
	{
		path2Root = path2Dirs[0] + dirSeperator.ToString();
		path2Dirs.RemoveAt(0);
	}
 
	// webChecking, checks to see if the hostname is the same, if it is 
	// it continues as normal, else it returns as if the root were different
	if( enableWebChecking )
	{
		path1Root += path1Dirs[0] + dirSeperator.ToString();
		path1Dirs.RemoveAt(0);
		path1Root += path1Dirs[0] + dirSeperator.ToString();
		path1Dirs.RemoveAt(0);
 
		path2Root += path2Dirs[0] + dirSeperator.ToString();
		path2Dirs.RemoveAt(0);
		path2Root += path2Dirs[0] + dirSeperator.ToString();
		path2Dirs.RemoveAt(0);
	}
 
	// If the roots aren't equal then the relative path IS the absolute path
	if( path1Root != path2Root )
		return path2;
 
	string filename = "";
 
	// Remove filenames
	//  String.Split will leave an emtpy element at the end if the string
	//  ends with the seperator, so we will always have to remove
	//  the last element
	path1Dirs.RemoveAt( path1Dirs.Count - 1 );
 
	filename = path2Dirs[ path2Dirs.Count - 1 ];
	path2Dirs.RemoveAt( path2Dirs.Count - 1 );
 
	// Worth through the algorithm
	while( 
		path1Dirs.Count > 0 && // so we don't get IndexOutOfRangeException
		path2Dirs.Count > 0 && 
		path1Dirs[0] == path2Dirs[0] )
	{
		path1Dirs.RemoveAt(0);
		path2Dirs.RemoveAt(0);
	}
 
	while( path1Dirs.Count > 0 )
	{
		relativePath.Append(".." + dirSeperator);
		path1Dirs.RemoveAt( 0 );
	}
 
	while( path2Dirs.Count > 0 )
	{
		relativePath.Append(path2Dirs[0] + dirSeperator);
		path2Dirs.RemoveAt( 0 );
	}
 
	relativePath.Append( filename );
 
	return relativePath.ToString();
}