Zoho Invoice - 100% free online invoicing software for small businesses.
Wherefour - Wherefour is an ERP (enterprise resource planning) solution used for managing customer relationships, and inventory, etc.
FreshBooks - The ideal accounting software for small business owners.
entree - Entree
Invoice Ninja - Invoice Ninja is a free, open-source solution for invoicing and billing customers.
Sage Intacct - Intacct is a cloud-based accounting and financial management system that is designed for the businesses of all levels.