What's your most reused class?

Question

Every programmer ends up with a set of utility classes after a while. Some of them are true programming pearls and they are reused in several of your projects. For example, in j

Answer 1

A utility class that has logging and email functionality. An extensions class that contains extension methods. A reporting class that basically harness the reporting services web service and makes it easy to stream reports as excel, pdf, etc.

Examples...
1.) Utility Class (static)

   public static void LogError(Exception ex)
    {
        EventLog log = new EventLog();
        if (ex != null)
        {
            log.Source = ConfigurationManager.AppSettings["EventLog"].ToString();
            StringBuilder sErrorMessage = new StringBuilder();
            if (HttpContext.Current.Request != null && HttpContext.Current.Request.Url != null)
            {
                sErrorMessage.Append(HttpContext.Current.Request.Url.ToString() + System.Environment.NewLine);
            }
            sErrorMessage.Append(ex.ToString());
            log.WriteEntry(sErrorMessage.ToString(), EventLogEntryType.Error);
        }
    }

2.) Extensions Class

   public static IEnumerable WhereIf(this IEnumerable source, bool condition, Func predicate)
    {
        if (condition)
            return source.Where(predicate);
        else
            return source;
    }