Knowpress is a nice clean knowledge base wordpress theme. It is commercial

QnA is a simple question and answer theme for your wordpress blog. It can be setup to work as a support or help page with FAQs.

Full wordpress ticket and helpdesk support theme for Wordpress.

Support is a helpdesk and customer support theme for Wordpress. It has knowledgebase and tickets providing a great customer support functionality. It is commercial  from woothemes.