Here are just a few of the "helpers" I frequently include in web applications to make life easier.

StringListFromDelimitedString

public static List<string> StringListFromDelimitedString(string commaDelimitedString, string delimiter = ",")
{
            List<string> rtnVal = new List<string>();
            if (commaDelimitedString != null)
            {
                rtnVal = commaDelimitedString.Split(new[] { delimiter },
                StringSplitOptions.RemoveEmptyEntries).Select(b => b.Trim()).ToList();
            }
            return rtnVal;
}

IntListFromCommaDelimitedString

public static List<int> IntListFromCommaDelimitedString(string commaDelimitedString)
{
    List<int> rtnVal = new List<int>();
    if (!string.IsNullOrWhiteSpace(commaDelimitedString))
    {
        List<string> stringList = StringListFromCommaDelimitedString(commaDelimitedString);
        foreach (var value in stringList)
        {
            int intValue = 0;
            bool parse = int.TryParse(value, out intValue);
            if (parse)
            {
                rtnVal.Add(intValue);
            }
        }
    }
    return rtnVal;
}

CommaDelimitedStringFromStringList

public static string CommaDelimitedStringFromStringList(List<string> stringList, bool spaceAfterComma = false)
{
    string rtnVal = "";
    string delimiter = !spaceAfterComma ? "," : ", ";

    stringList.RemoveAll(o => String.IsNullOrWhiteSpace(o));
    rtnVal = string.Join(delimiter, stringList);
    return rtnVal;
}

StringFromQueryString

public static string stringFromQueryString(string queryString, string defaultValue = "")
{
    string rtrnVal = defaultValue;
    HttpRequest request = HttpContext.Current.Request;
    if (request.QueryString[queryString] != null)
    {
        string queryStringValue = request.QueryString[queryString];
        rtrnVal = queryStringValue;
    }
    return rtrnVal;
}

GetRootPath

public static string GetRootPath()
{
    return HttpContext.Current.Server.MapPath("~");
}

FileExists

public static bool FileExists(string fileName)
{
    // note: all paths should start with a character and end with a "/"

    bool rtnVal = false;
    string rootPath = HttpContext.Current.Server.MapPath("~");
    string fileSpec = string.Format("{0}{1}", rootPath, fileName);
    if (File.Exists(fileSpec))
    {
        rtnVal = true;
    }
    return rtnVal;
}