WHMCS!

Enough said. The features speak for themselves. Also, the fact that it can integrate seamlessly with your existing hosting template, it works like a charm.

I say this from experience. I personally used ClientExec and it's terrible. Just the fact that it's so, how do I put this, ugly. Bear in mind that it has the same features as WHMCS but it still sneaks somewhere behind WHMCS.

Also, MANY hosting companies use WHMCS so you can purchase/use in confidence.

Best regards,
David Vazquez