One of the market leader who made automation of web hosting easy is WHMCS. The name stands for Web Host Manager Complete Solution.
1. Domain Tools
— automatic domain name check and registration, transfer, renewal, DNS management.
2 Account Management
— accounts creation, upgrades, suspensions, mail notifications.
3. Web Hosting Tools
— server, cPanel, Plesk integration, SSL certificate provision automation.
4. Payment Integration
— invoicing, collecting payments, taxes, sending reminders, PayPal, Authorize.Net integration.
5. Support Tools
— ticketing system, knowledge base, downloads, Twitter account integration.
WHMCS has modular structure and could be easy extensible, plus it has well documented API support.
You'll need to pay for a license to use WHMCS. There are three types of subscriptions starting from $18.95 and special starter plan for $15.95.