STAKADUSH  All I can say - if you need a time tracking application try this one first. It will save you time in trying other ones. :)  
(Version 3.0)

ROBOMALO  This is a great program, but 3.0 has been promised for a LOOONG time.  
(Version 2.3.1)

JOESHMOE200US  Simplest and inexpensive Job Time Tracker. I use this to quicklly keep track of and to e-mail out invoices for my web design business. I tried out demos of about 5 other time trackers, and this had a great interface and allowed me to filter in completed jobs by dropping them into a folder I created. The function that allows you to create a PDF, or to e-mail directly is a huge time saver.  
(Version 2.3.1)

ERICG  Wow, just tried it and loved it. I've tried other Mac Project Timers and keep on encountering limitations. Everything about this application is elegant - the timer, the invoice, the ease of use. Just outstanding.  
(Version 2.3.1)

SARASWATI  I used MultiTimer Pro (OS 9) for many years and loved that app. Unfortunatley, the developer decided not to create an OS X version. On the Job is a terrific substitute MultiTimer Pro. In fact, the developer, Dan Messing, also used MultiTimer Pro for years and was inspired to create On the Job. This app is elegant and easy to use. I totally concur with the previous reviewers. Thank you Dan Messing!  
(Version 2.2.2)

STARMANTA  This is a very slick app! As another reviewer said, everything about it screams "polished".

I have only one issue with it, and that is the (apparent) inability to set different rates for different tasks within the same project. For example, one might invoice $25/hour for coding but only $12.50/hr for reading the related material.

This is a minor gripe, though, and probably one that most people will never notice.  
(Version 2.2.2)

ROBIN-O-HOOD  On The Job is so easy to use that I actually enjoy putting it to work tracking and billing client projects.

I also use it to track the time (by task) I spend on my own project developments as it helps me to clearly see where my time goes. A great professional & personal time management tool.

See how it works (with illustrations) and a detailed review @ The Apple Blog:

http://theappleblog.com/2006/09/30/on-the-job/

Steven Hotelling  
(Version 2.2.2)

NONEO  Freelancer's first class choice!

This app is excellent, it never let me down!

It covers all I need, a great timer, creates invoices and looking beautiful not to say sexy! ; )

Respect to the Developer! Thanks!  
(Version 2.2.2)

AUDIOWIZARD  This is the best invoice app I've come across, I've looked at many, including TimeNet, Invoicer, CGInvoice, iBiz, TaskTime, StudioCzar, and a few other's I've already forgotten. The only downside is it's customization is fairly limited, being a super basic app. I like basic/clean apps, but this one is slightly too basic. I imagine as the developer continue's to make improvements and updates, this will shape into an absolute solid app to use for easily/quickly making invoices so that you can get back to what you enjoy doing on your mac, which probobly isn't learning new business software!   
(Version 2.2.1)

ADRIAN L. D. SAMPSON  Finally, a usable invoicing application. This application is full-featured but does not sacrifice polish or intuitiveness. The interface reflects its straightforward operation that integrates line-items with timed tasks and makes printing attractive invoices mindlessly simple.

Like most such applications, On The Job includes a task timer; the idle warning (if you stop working but forget to stop your timer) is a godsend. This software does exactly what I, at least, need in an organized, logical way.  
(Version 2.2.1)