Working on the RememberTheMilk.Net required a hash function identical to the PHP md5(), and here's the code:

Private Function getMd5(ByVal password As String) As String 
  Return System.Web.Security.FormsAuthentication.HashPasswordForStoringInConfigFile(password, "md5").ToLower
End Function

Very simple, but still required a little google work.

Remember to reference System.Web.