Workrave is a free (GPL) app for Linux and Windows that reminds you to take typing breaks (with configurable timing, of course).
I found out about it from the Gnome-announce mailing list, of all places. Obviously, their focus was on the Gnome/Linux version, though I’ve used just the Windows version so far. Unsurprisingly, because the Windows version is based on the GTK toolkit, it’s still very… Linux-looking ;).
If you care about preventing RSI, it couldn’t hurt to give Workrave a try. By default, it comes configured to suggest a 30 second “micro-pause” break every 3 minutes, and a 10 minute rest break every 45 minutes (those numbers are from memory, but I think those are about right).
I’m not expert on typing-injury prevention, but that seemed awfully frequent to me (?). If anyone can speak to what kind of numbers are suggested in the scientific community, let me know (for all I know, perhaps that’s how Workrave derived its defaults, but I’m not sure). In the meantime, I’ve configured to the micro-break interval for 10 minutes, which seems a bit less intrusive.