Once installed, Pomodoro Desktop becomes a menu bar item. You'll see a timer set for 25 minutes, with is the length of one "pomodoro," or block of time during which you're meant to work steadily. Click the timer and you'll see several options, including "Start pomodoro." A voice counts you down ("On your mark, get set, go") and you're off. From there, the timer counts down until the 25 minutes is gone and you've earned a 5-minute break.
That's very nice, but I had to make a few adjustments. Select "Preferences" to open the preferences pane. First, Pomodoro Desktop plays a ticking sound as you work. I found it to be extremely annoying, so I de-selected the box to disable it. Next, I disabled the voice that gets you started. To do this, go to the Speech tab and de-select that option. While there are many voices to choose from, some less Robbie-The-Robot than others, I had to kill that feature entirely.
Other options include Growl integration which is nice, Twitter integration which is odd (I doubt my friends care about what project i'm working on, but co-workers probably would), full support for AppleScripting and customizable keyboard shortcuts.
Another nice feature lets you review all of the "pomodoros" (or work cycles) that you've completed in a day or even since you started using the app. For the whopping price of free (donations encouraged), you'll be hard-pressed to find a better implementation of the Pomodoro Technique for that Mac than Pomodoro Desktop. It does what it should while staying out of the way. Check it out.