Meditation or mindfulness timers can be used to time meditations of various lengths. They are also quite handy as periodic reminders to pause, relax the eye muscles, take a few deep breaths, stretch... The one listed below are free to use online or download, except for the last one.
OnlineMeditationTimer.com: Online version by creator of Meditate iphone app
Online Mindfulness Bell: Online version
Your Meditation Timer: Online version or make your own MP3
Mindfulness Clock for PC: PC download version
Quikalert: Mac download version
Prodme: Mac download version
“Meditate” iPhone Application: $2.99+