I am using Cent OS 5. I am looking for an app which will alert me after some time to take break. So I can use computer as healthy computer usages ?
the best choice for you would be probably Workrave
Here are two more alternatives that haven't been mentioned in the other answers.
Xwrits reminds you to take wrist breaks, which will hopefully help you prevent repetitive stress injury. It pops up an X window when you should rest; you click on that window, then take a break.
Repetitive Strain Injury can occur as a result of working with a mouse and keyboard. This utility can be used to remind you to take a break now and then. It displays a random picture from a collection at specified intervals for defined durations. You can configure the pictures to show and lengths of time to show them. You might use the breaks reminders to do some stretching exercises, for example, or as a reminder to walk away from the computer for a while.
If you are OK with using a "web app", Evo will do. May not be idea for several situations, but if you spend a good deal of time in a web browser, it should fit just fine.
Maybe Project Hamster?
Lifehacker had it as one of their top five time management programs.