Metronomics by John Nastos is a great metronome app for the iPhone designed with real musicians in mind. This isn't the kind of metronome geared towards beginner who's still learning his or her instrument or how to keep basic time, though it's certainly capable. It's geared toward working musicians like John who can use it as a serious practice tool and to stay on top of their game between gigs.
You can set the app to do any time signature in any tempo and assign a variety of sounds to the different subdivisions, but where the app really shines as a serious practice tool is the ability to randomize the beats. Want to hear something in 11/4 at 150bpm with occasional 8th-note triplets sprinkled about? I thought you might. The result isn't always supremely musical, but it forces the practicer to be on top of what's going on and provides better "real-world" practice than a standard metronome that clicks on every downbeat.
The video does a better job of demonstrating all the features and how to use them:
If you're a serious, or perhaps even simply a semi-serious musician, this app is a steal at $2.99.