If you want an Agile tool that would get all project management things done for you, is simple to use, and very powerful (this requires careful thinking to be put into it and a TON of reading on Project Management Principles) is Trello trello.com
For customers, you can create multiple boards and manage it from there.
You can also look at the other options (may be paid):
Atlassian JIRA
Basecamp
Free Desktop Softwares (for building Gantt charts):
LibreProject
Really good, self hosted project:
DotProject (PHP based, development abandoned)
For billing:
Unfortunately, I have limited exposure to these type of tools, but, I would think, plain old Excel. ;-)
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If you want an Agile tool that would get all project management things done for you, is simple to use, and very powerful (this requires careful thinking to be put into it and a TON of reading on Project Management Principles) is Trello trello.com
For customers, you can create multiple boards and manage it from there.
You can also look at the other options (may be paid):
Really good, self hosted project:
For billing:
Unfortunately, I have limited exposure to these type of tools, but, I would think, plain old Excel. ;-)