Micropauses are short, regular pauses during your work day. They are especially important for people who suffer from wrist pain due to working in front of a computer all day. There are free and commercial options available for Mac, Linux and Windows.
I prefer simple so my favourites are AntiRSI for Mac and Dr. Wright for Linux.
AntiRSI (Mac - Open Source) - A simple program similar to !DrWright, works quite well and supports micropauses as well as longer work breaks.
Dr Wright (Linux/Gnome - Open Source) - A newer Gnome program, very simple but prettier and easier to configure then Xwrits. It appears to be bundled as part of the default Gnome setup these days.
RSIBreak (Linux/KDE - Open Source) - Supports short and long pauses and shows exercises during long breaks (or will soon).
Strech Break (Windows & Mac - Commercial) - Primarily aimed at kids, teaches how to stretch in front of a computer. There's a free version.
WorkPace (Windows, Unix & Mac - Commercial) - A little clunky in the GUI but a very nice client which supports work breaks, micropauses, typing statistics and shows stretches during breaks. I bought it and the couple problems I had under OSX were quickly resolved by their helpful support people.
WorkRave (Windows and Unix - Open Source) - The most complete free alternative. Supports many types of breaks and shows stretches during breaks.
Xwrits (Unix - Open Source) - The traditional Unix program that has been around forever. It works well but is ugly and doesn't support micropauses directly.