18 years ago, before Tonya and I started TidBITS, I was working as a Mac consultant and tracking my time with a program called Timeslips (see "Slip Slidin' Away," 1992-02-17). I remember it as having more features than I needed, features which consequently made using the program somewhat clumsy. Timeslips exists only for Windows now, but there's a brand new Mac OS X-native program called Billable that looks clean and easy to use. With Billable, you can set up clients with individual hourly rates, track the services you perform for those clients on an hourly or flat fee basis (Billable includes a timer for tasks performed at the Mac), and generate PDF or plain text invoices based on the services you've performed.
Billable 1.0 was initially released in September 2006, and the new Billable 1.1 makes it possible to add taxes, generates invoice numbers according to your scheme, increases the customization options for invoices, and automatically saves and backs up data. You can export the data to XML, making possible communication with other applications, such as your accounting package. Billable is worth a look for any Mac user earning a living billing by the hour; be sure to check out Clickable Bliss's well-done screencasts explaining Billable's operation and features.