Depending on your level of complexity you need there are two projects to look at.
http://drupal.org/project/support - Gives you a full fledged ticketing system in drupal. And lets you set up multiple clients,etc. Has way more than most people need and requires a bit more overhead to set up.
http://drupal.org/project/casetracker - I use this for exactly the purpose you are talking about. It is a simple casetracking tool which just allows priorty and status fore each case which you then comment on. Although it has notifications integrations, I actually use it with the subscription module to get the email as well when my clients post a case.
Take a look at http://forenasolutions.orghttp://forenasolutions.org/ (you'll only have view access to the cases). Add the content_access module if you need extra security and you're good to go.
Dave
________________________________ From: support-bounces@drupal.org [mailto:support-bounces@drupal.org] On Behalf Of lamp@afan.net Sent: Friday, March 29, 2013 8:12 AM To: support@drupal.org Subject: [support] Drupal 7 as simple Project Manager
Hi, I have few "clients" I'm helping them with their website. They are sending to me stuff to add/delete/edit on my private email and it's now a little mess. And I overlooked few times their emails. I was thinking, if it's possible, to build Drupal 7 website (I have few "empty" domains) and use it as a simple Project Manager. My idea was to create accounts for each client and they will be able to see only their stuff. I would create content type "Task" where they can write down what they want me to do, upload files, etc.
How hard/easy it would be to make? Any comments?
Thanks, LAMP