What is the help system, web-support desk that you use to support your clients?

More specifically, if do your clients click on "help" or something from your webmail, that takes them into the support ticket page?   Reason I ask is.. looking for a solution so that users do NOT have re-login or re-input their email and credentials in order for complete a ticket request.

We have Smartertrack, but that doesn't allow for users that are in webmail.xxxxx.com   to login into our support.yyyyyyy.com  (in order to use Sos - has to be same domain).

Just curious what others are doing for support /ticketing platforms .. and how they are implemented.