Here's a simple critical hit system of brevity and non-damage amplification.
Simple critical hits:
For any strike that can hit on a d20 roll of 19 or 20- roll 1d6 and consult the chart:
1-2: Staggered- The blow knocks the defender off balance. Additional attacks against the target this round are at +2 to hit and the foe suffers a -2 to any attacks they have left this round. The defender may be pushed 10' by the attacker (who may follow up as free movement) if the the attacker is of at least equal strength or of a larger size.
3-4: Stunned- The blow stuns the foe who is unable to act meaningfully or move under their own direction for the remainder of this round and all of the next round. An ally may help the defender move 1/2 their movement rate. The defender may not cast spells or use magical devices while stunned and attackers may get bonuses to their attacks vs a stunned defender.
5-6: Clobbered- the blow wounds the defender so as to reduce their movement rate by 1/2 and cause them to suffer a -4 penalty to attacks and saves until the wound is healed.
Some foes are immune to specific effects, or may get a save vs some critical hits.
For a really cool use of other RPGs properties in D&D-like games as they relate to Critical Hits (and other areas) check out this post on using Boothill rules in D&D.