What are the tools that really helped your store?

What are the tools that really helped your store?
Some categories to make it easy (Just pick a category below or any tools you'd like to recommend)

1. helped you with "Leads"
2. helped you with "Customer Service"
3. helped you with "Transactions"
4. helped you with "Outsourcing"
5. helped you with "Tracking Data" (Accounting, Contacts, etc..)