I've had this sitting around for ages, but didn't release it as I didn't really think it fit in with any of my other plugins. Decided to release it, as AFAIK there isn't one out there.
Admin command:
sm_crits *target* *0.00-1.00* - Sets a target's crit chance. 0 returns it to normal. It is a decimal e.g. 0.5 is 50% chance.