Simply Invoices is a new web-based invoicing tool built to work with Basecamp. If you're looking for a quick and easy way to invoice your Basecamp time, Simply Invoices is worth a look.
There's an informative screencast (2:30) and a step-by-step screenshot tour at the site. They both do a good job of showing you exactly how it works.
Other great invoicing tools that work with Basecamp include Blinksale and FreshBooks. You can find even more products that work with Basecamp on the Basecamp Extras page.
Now if only there was an app that could make all your clients pay those invoices on time.