timer-appletfills this purpose quite nicely. It's part of the Gnome desktop though not it's not installed with Ubuntu by default.
The timer applet docks on the Ubuntu toolbar, out of the way but easily accessible when you need it. Click it to open its dialog box, where you specify how long a period you want. Periods are either entered manually or via customizable presets. Once started, the timer applet shows you via the toolbar how much time remains.
Right-click on the applet icon to reveal its Preferences menu. One thing you'll want to do is change the sound. The default alarm is simply far too soft to hear. I much prefer the loud and startling old-time phone ring from
To install the timer applet:
sudo apt-get install timer-applet