I've tried a lot of online time-tracking solutions and have personally come to love Harvest for its simplicity, good looks and -- more than anything -- the great tools they provide to make punching in and out simple enough that people might actually do it. They've had a Dashboard widget (also available for Yahoo! Widgets) for some time which allows for insanely fast job tracking, whether entering it in post or starting and stopping a timer. You can even use Twitter from your phone to stop a running job timer if you forget. As of last Thursday, though, you can handle all of your time tracking and expense logging through a spiffy iPhone interface.
I have high hopes for the opening of the App Store and the veritable bounty that the Mac developer community will be able to offer. In the meantime, I'm really enjoying the number of highly-useful sites that are offering iPhone portals, and the companies and developers that are stepping up to provide services for a device which they must agree is not a passing fad.