I agree with the tutorial basic crit system is easy, but I rather mean something more advanced and most importantly well written, because it would check every dealt hit on server, so someone unexperienced can lag server I guess, or just waste memory.
You can expand this script nicely with some interesting ideas, let's say playing with elemental damage, for example add bonus only to fire damage after some sort of fire quest or bringing specific item to NPC, so mages/druids can choose which "element" they want to develop on their char. It could give diversity. More or less like skill tree, but in this case related only to critical damage. It could be also related not only to crit, but every damage you deal. Longer scripts wouldn't be so simple anymore.
Anyway, it is just an idea