[1gor]

http://www.redmine.org is Ruby on Rails -based project management app. You can configure it to run your code development projects (it basically an improvement on http://trac.edgewall.org, features wiki/tickets/milestones/integration with git/svn... etc ) and set up a separate customer support section.

They have email gateway and a very flexible user rights management features. You could set up customers to email you issues which will automatically become tickets in your development project (which is accessible only for developers). You can also set up a wiki available only for customers.

Since you are (hopefully) already running issue tracker/version control environment for developers, you can thus avoid creating a redundant extra layer of 'support ticket' database and 'customer knowledge base' altogether.