HelpNDoc - What is HelpNDoc ? Easy to use yet powerful help authoring environment which can generate various documentation formats from a single source.
Freshdesk - Freshdesk is a cloud-based customer support software that lets you support customers through traditional channels like phone and email, social channels like Facebook and Twitter, and your own branded community
ManualsLib - Just enter the keywords in the search field and find what you are looking for!
Zoho Desk - Industry's first context-aware Helpdesk Software
Manuzoid - Repository that includes user manuals for various products
Salesforce Service Cloud - Service Cloud integrates customer service with social networks and provides a self-service customer portal and knowledge base system.