I am looking out for an app (for Mac OS X) that reminds to take a break (gives out a message) at pre-scheduled time.
Free (or low cost) app will do, as I don't want to spend my hard earned cash on this simple app.
An application that I use to be aware of the time on the computer is awareness it don't put a message only a sound indicating the time you have been using the computer without a break.
There is one such tool I know, called (unsurprisingly) Time Out. It's free as in beer and seems to be well-maintained.
If you just want to limit the time you spend online, you can use the freely available app Freedom.
AntiRSI does what you want.
I was doing a quick google search and this question came up. You can also try Rest, which is a break reminder app that can also suggest exercise suggestions.
Disclaimer: I made it.