I used Billings for a while, but it doesn’t have multi-user support—so if you have partners or employees it’s a no-go.
I tried Blinksale, but it lacks time tracking so I moved on.
I then switched to FreshBooks and used it for a year and a half, but recently quit them as I was frustrated by the clunky UI, slow development, and crappy invoice design. In my view, the FreshBooks development team isn’t particularly responsive—over the time I used the service, they seemed to focus on features that would sell the service to new customers rather than working to serve their existing customers.
I recently signed up with Cashboard, and I’m really impressed. The interface is modern and very usable, there are essential features but no bloat, and most importantly, the developers are very responsive and always working to improve the system. It has time tracking, estimating, and invoicing, & has a desktop widget timer. You can also integrate Cashboard with Basecamp.
Cashboard allows you to customize your invoice and estimate templates, which is essential for a design firm. The first time I created an estimate my client remarked on how professional it looked. You can choose to send invoices & estimates via snail mail (something FreshBooks still hasn’t gotten right), which is great for your old-school clients.
The cost is reasonable, as it’s based on how many active projects you have at once. There’s a free trial and a limited free level as well.
I’m not affiliated in any way with Cashboard, but as you might guess I’m a real fan!