Anne rule by St. Matty. Returne to the Maine Page.
3d6 vs. 1d20
When rolling an Attack, players may choose to roll 3d6 in place of 1d20.
The following results are used:
⚀⚀⚀ ^{1}/_{216} chance 
automatic miss.

⚀⚀⚁ / ⚀⚀⚂ / ⚀⚁⚁ ^{1}/_{24} chance 
automatic miss. 
⚁⚁⚁ 
roll 1d2 and add that to the attack 
⚂⚂⚂ 
roll 1d3 and add that to the attack 
⚃⚃⚃ 
roll 1d4 and add that to the attack 
⚄⚄⚄ 
roll 1d5 and add that to the attack 
⚃⚅⚅ / ⚄⚄⚅ ^{1}/_{36} chance 
automatic hit.

⚄⚅⚅ ^{1}/_{72} chance 
automatic hit.

⚅⚅⚅ ^{1}/_{216} chance 
automatic hit.

This grants you approximately the same 1/20 chance of critical success or failure in your attacks – actually it's a little lower, 1/21.6, but the rewards and punishments are scaled to compensate.
Effect Progression
The progression for negative effects is essentially:

However it can be tweaked based on particular powers. For example, "immobilised" may be upgraded to "restrained". Not to mention blinded, deafened, dominated, weakened, etc.
The progression for positive effects is trickier, and can be divided into three parallel progressions, each of which can lead to any of the others in various circumstances:

Chance to Hit
Note that when rolling 3d6, the chance to hit a monster may be dramatically reduced. You can see in the following graph that: if you need to roll better than 11 to hit a monster, you're much better off rolling 1d20 than 3d6. This can play into combat, for example players making knowledge checks to determine what to roll, or learning by experience.
Taking this graph into account you may choose to instead use 3d6 + 2, which lets you hit anything that requires up to 15 as easily as 1d20, and lower monsters (up to around 12) much more easily. Given that, in this system, 18, 19, and 20 are variously critical hits, this means that you only suffer against monsters that require 15, 16, or 17 to hit... and you should probably flee such an encounter anyway.