As also found in the official Google Android Market:

This application is a call minutes, sms, mms and data usage meter to monitor your credit usage. Never exceed your quota again!

Supporters unlock: free times/contacts/groups; widget; usage log; greater configuration of plan inclusions + more. A free 5 day trial is offered.

Please report bugs!

Please visit http://creditusage.matthewwardrop.info for more information.