We also have "fumble", meaning sometimes when you roll 1, you get to re-roll, with 20 (or sufficiently high roll) negating the original 1 and possibly having some additional effect. Not sure how fumbles are in DnD... we just use the name for a specific thing for some reason. Back when we played DnD for a short while, we didn't use any optional/additional rules.
The comedy 1s, fumbles and 20-20s add to our game are worth it though. Our game is not a parody and overall lacks humor but our own antics create that, to balance the general dark feel of the setting. It is always ridiculously amusing when certain member rolls 1, then under 10 with D20 and has his axe fly toward something (statistically unusually many times toward my character, once my character almost lost an arm to that). There has been a couple of awesome kills that way. And despite being silly, we worked this into our game and now that member has some ability to throw things effectively.
(The original reason for throwing the weapon on critical fails is that the character lacked axe skill, and due to several reasons, he cannot learn the skill anymore, though he does have kind of an advanced skill with it... It is complicated.)
Our GM's homebrew system was pretty rudimentary at first, and it has been developed as the game went on. Now it is rather odd, compared to commercial systems, probably. Most certainly it is something that is impossible to write down and try to sell to others, it is rather unintuitive without living two years with it.
One good thing about it is that since it is a system without classes (kinda, effectively every player is their own class and our GM has templates for enemies), melee characters and wizards etc. are all equally viable, in theory.
In practice, this depends on players imagination, as we get to create our own skills, abilities and spells (everything's cleared with the GM, of course), though our GM has granted us some abilities he came up with, usually due to story events. If skills aren't quite useful enough, we usually tweak them as we go if necessary. Since our game is really more about role-playing than rule-playing, everyone sticks to their character's class (or perhaps theme is more apt) quite faithfully.
