Tried a lot of different billing software, as others have already posted the different ones and came away using WHMCS because of its extremely easy to use, but yet still powerful.

Support is superb for the software every question I have had to ask with their support has been answered the same day with some of them answered within 30 minutes or less even on the weekend which is amazing! I didn't expect a reply till monday! Plus features are added all the time that are most requested from their users.