RSIGuard

• An automated (and highly configurable) break timer, which measures how much you use the keyboard and mouse and correlates this with data you provide on how painful typing and mousing are.  It will recommend you take a break when your accumulated keyboard and mousing strain reach certain levels.  It can be configured to disable keyboard and mouse input to any other programs during this time, to enforce the breaks.  AlexChurchill finds this very useful.
  
• It shows stretches during breaks, with description and animation.  These have a very high overlap with the anti-RSI stretches AC was recommended by a physiotherapist.
  
• AutoClick? feature.  Some people love it, some people hate it.  But you can't deny that when you're using it successfully, it really reduces mouse clicking strain.