If you need to track billable hours and you want to do it using a client with clear UI, then Billable is for you. Clickable Bliss' first product lets you keep track of clients, has a built in timer to keep track of your time (duh), and support for different billable rates per client. It can also create PDF invoices with your information already filled in.
Until October 31st you can get Billable for $19.95 (after that it goes up to $24.95), and there is a 30 demo available.
Still not convinced? Check out the screencast.