cpResellerClients is a PHP5 class that provides information about cPanel clients under a reseller account. It can be used as a convenient alternative to logging into WHM to list the accounts, and can also combine the clients of more than one reseller account into a single list.
Custom templates can be used to present the data in any form that suits you, for example as an HTML table or plain text for sending via email. The class includes a method to only show clients who are over a certain percentage of their disk quota.
You could set up a cron task that uses this class in order to monitor the growth of each account over time, or perhaps to send custom emails to clients when their account reaches a certain limit.
The information available from the class is the same as that shown in WHM's "List accounts" function (or more specifically the fetch CSV option on that page); that is, Domain name, Username, Contact email, Server name, Server IP, Setup date, Partition, Quota, Space used, Package, Theme and Owner.