The download link above is broken, due to the developer closing down. Use the following link instead or Google "coffee break pro x".
Even though Coffee Break Pro X is no longer officially supported, it is still one of the best break reminder and RSI-prevention programs. I like it because it really forces you to take your break: the break happens and there is no way to get out of it; you can't hide, quit, switch to another program, or anything (you can force quit it, though).
The program sits in your dock, with or without the time remaining until the next break, or you can optionally have a small floating window to display the time instead. The main feature is the breaks; you can customize them from hours to minutes to seconds, and even have "micro-breaks" interspersed between your main breaks. You can customize the screen that appears during a break using jpeg images of your choice, simply by dropping them into the "Users/username/Library/Coffee Break Pro Images" folder (Coffee Break Pro will cycle through them, like a slideshow), as well as adjust the transparency of the image. It has options for alerting you ahead of your upcoming break by bouncing the dock icon, displaying a window, or playing sounds so it doesn't come as a shock or when you're unprepared. The program also features the ability to sense activity/inactivity of your mouse or keyboard actions, so it can tell if you've left your computer regardless of the break schedule. Finally, it is capable of running AppleScripts at the beginning or end of a break, which allows for considerable expansion, simply by placing the files in the "Users/username/Library/Coffee Break Pro Scripts" folders.
To boot, it has a detailed help package, accessible by the Help menu (cmd-?).
A helpful hint: Drag the application icon from your Applications folder into the "Login Items" list in your Accounts pane in the System Preferences. This will allow Coffee Break Pro to start up every time you start your computer.
I really recommend this software, even though it has unfortunately been discontinued. I'm using Mac OS X 10.4.11 Tiger on a G4 PowerPC and it runs perfectly.