A few Web 2.0 apps I've noticed recently

• SlimTimer is a time-tracking web site.
• BlinkSale lets you send invoices. You can import data from Basecamp.
• Cashboard lets you track time and send invoices. It integrates with Basecamp.
• Freshbooks also has time tracking and invoicing. It can also do online invoicing/payment. It integrates with Basecamp.

There are probably plenty of others, but anyhow. [edit: like the FOSS BambooInvoice for CodeIgniter]

BTW, is everything Web 2.0 written in Ruby on Rails lately? :)