If you are using WHM combined with WHMCS you can surely do this.
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My vote has always went to WHMCS since it was first released. I've never been a big fan of ClientExec as I've always found it to be much slower and really, more of a hassle. WHMCS is simple, straight to the point and efficient.
Another vote for WHMCS, they just released a new version. WHMCS rocks, I've never used a different billing system, and I don't plan to switch.
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One other option is called cPCreator, although their site seems to be undergoing maintenance. http://www.cpcreator.net/ . Unlike ClientEcex or WHMCS it only works with cPanel/WHM Based Servers, but hey, it is free. It looks like WHMCS as well.
With WHMCS everything is completely automated. The user orders and it automatically creates the account, username/password, etc, then send the user a confirmation with everything. It is definitely worth it, a lifetime license is a little over $300, and alot of resellers include it free.