Billable is the app I use from day to day. It includes an integrated job timer, so you can start it when you begin a task, and stop it when you end it. It lets you keep track of clients, even assigning a photo to each.
Billable does a few things well: invoices are customizable based on an HTML template you can edit. This is great for designers, like myself, who want to have a consistent face for their business correspondence. Also, it handles my obscure invoice numbering system with remarkable aplomb.
On the downside, it doesn't manage estimating, and lacks support for common task sets that can be applied to many different clients. Also, multiple-item editing is hard to do (at best). If you mistakenly input a task for the wrong client, you can't just drag the task to the correct one: you have to delete it an enter it again. Poopie.
Billable 1.2.3 is $34.95 from Clickable Bliss. It's universal binary, works with Mac OS X 10.4.4 or later, and features a fully-functional 30-day demo.
Like Billable, Fanurio includes a job timer and client tracker. They'll also make a custom template for you, based on a design you send them, as a free service.
In addition to Billable's feature-set, Fanurio includes multiple currency support and a healthy reporting tool that answers questions like "what have I done today?" and "who was my best client this year?"
Also helpful is the job timer's built-in reminder function. If you leave your computer idle for a certain amount of time, it will ask if you want to pause the timer. Perfect if you -- like me -- are easily sidetracked.
Fanurio also runs on Windows and Ubuntu, if you find yourself changing operating systems. You can migrate data from one installation to the next, but you can't (for example) sync between many computers.
Fanurio was updated to version 1.11 yesterday, and is $39. Their website says they're planning a new version for mid-September, priced at $59. It runs with Mac OS X 10.3.9 or higher, including Leopard.
On The Job
On The Job is a much simpler job timer app that records tasks and saves them in a list. It also lets you generate PDF, XML, CSV, or text invoices. When I used to work at my agency job, I used On The Job to record my time so I could input it into Clients & Profits.
On The Job lets you organize timesheets into folders, which is great for separate jobs for the same client. I was able to add a note for the items that I'd entered into C&P, which was a nice visual way to remind myself that I had to do my timesheets.
The software is incredibly easy to use, and includes keyboard shortcuts to start and stop the timer. Like Fanurio, it includes an warning that pops up if you leave your computer for too long, and gives you the option to subtract the idle time.
On The Job 2.3.1 is $24.95, Universal Binary, and requires Mac OS X 10.4 or higher. Discounts are available for two or more licenses. Also, if you buy now, you can get a free upgrade to version 3, which Stunt Software says is "coming soon."
WorkTimer, like On The Job, is a simple timer app. In addition to easily keeping track of job time, you can export the list to an XML file to use in other applications.
The app organizes projects as "documents." Despite the naming convention, the nice thing is that you can move tasks between documents by cutting and pasting.
This app is not to be confused with Work-Timer (note the hyphen) which is very similar in style and scope. However, Work-Timer is $15, and this app is free. Also, a commenter over at VersionTracker noted that Work-Timer's timer actually loses a second for every minute clocked. Yikes.
WorkTimer (sans hyphen) is free under the terms of the GPL, universal binary, and works with Mac OS X 10.4 and later. You can also download the software as an Xcode project, and tweak it yourself.
If you're not big on having an app to do your time tracking, you can opt to use the cloud (urp) instead. Tempo gives you a variety of methods to enter your time, and collects it all online. You can then view reports based on what you enter.
With Tempo, you (and a team of people, even) can enter your time with a Dashboard widget, via SMS or Twitter, or through your web browser. Tempo also has an iPhone-optimized website that works great in MobileSafari, or as a Fluid menubar app. And if that isn't enough, they have an API, so you can feed data to Tempo however you like.
Tempo features hefty reporting tools, to help you track how much time you're burning, or if your team-members are under-utilized. Tempo works with Basecamp, so you don't have to re-import your user list, either. For invoicing, Tempo makes data ready to be imported by Blinksale or QuickBooks.
Tempo has a free version, which lets you track time for one person and one client. They have plans ranging from $5 to $49 monthly depending on how many clients you have, and the size of your team.
LiveTimer is much like Tempo, and lives online, rather than on your desktop. It allows you to enter time via the web, an iPhone-optimized web page, or through what they call the "time ledger": a bulk entry page for when you forget to enter your time for a week. Or a month. Yikes.
LiveTime includes reporting, including the ability to save reports that you create as templates for later. Also, your data stored with LiveTimer is available through an API, so you can do with your data as you wish from any networked application. LiveTimer lacks any built-in invoicing tools, though you can export data as CSV or XML files to import into your accounting software.
LiveTimer is $5 per active user per month. Meaning, for example, if you have five active users, LiveTimer is $25 monthly to use. If it's just you, it's a slim fin. Discounts are available for non-profit organizations, and 30-day trials are available for everyone.
Now, if you're looking for a really robust solution to track your time and invoice clients, FreshBooks is a viable alternative to QuickBooks. It's far more than I'd ever need, but for some, it's just right.
FreshBooks lets you keep track of time via a web browser or a Dashboard widget. It handles estimating, expenses, invoicing, and even collections via online payment. FreshBooks even sends out paper invoices on your behalf.
Since its feature set is a little beyond the scope of simple timekeeping apps, I'll let you go take the tour yourself.
FreshBooks has free plans too, for three clients and one user. After that, costs range from $14 to $149 monthly.
Over to You
I know what you're thinking: Where's QuickBooks? The truth is, it's way more than I'll ever need for billing software, so I don't feel like I have an educated opinion about it. For a sole freelancer just starting out, QuickBooks seems like overkill. I know it's the 800-pound gorilla in this field, and at $199 per copy, it's far more expensive than most of these options. But if you love QuickBooks, make your case in comments.
For everyone else, I'm almost certain I've completely omitted your go-to timekeeping and billing app, as there are so many to choose from. So feel free to share your favorites, and how you use them, in comments.