I use fumbles in our game. They do not happen very often and have added to what happens story wise. I have had one player changed the direction of his character development because of the unique ability to always end up getting the same result. It works as follows:
If you roll a 1, there is a fumble check which is a DC 10 Dex check. You also lose any further attacks if you get multiple attacks (if this occurs on your turn, or further AoO if you get multiple ones).
If you fail the Dex check, a critical fumble has occurred and is essentially rolled against a table I found a long time ago somewhere. Except I did something different with the implementation of the table. It was a d100 table and it just took too long to 1) find it in the middle of my stuff and 2) roll against it and look it up.
So, I converted the table to 40 entries and printed each entry on a business card (using Avery cards). I put the word FUMBLE on the back of the cards. Now all I do is shuffle them before the game and put them on the table. If someone gets a critical fumble, turn over the top card and you have your result. Sometimes additional rolls are required by the cards.
Here are the card descriptions. The number in front is the number of them in the deck:
2 - SLIP
Make a DC 10 Dexterity Check
If the check fails, you fall down stunned for 1 round
2 - SLIP
Make a DC 10 Dexterity Check
If the check fails, you fall down stunned for 2 rounds
2 - SLIP
Make a DC 10 Dexterity Check
If the check fails, you fall down stunned for 3 rounds
2 - SLIP
Make a DC 10 Dexterity Check
If the check fails, you fall down stunned for 4 rounds
1 - STUMBLE
Make a DC 15 Dexterity Check
If the check fails, you fall down stunned for 1 round
1 - STUMBLE
Make a DC 15 Dexterity Check
If the check fails, you fall down stunned for 2 rounds
1 - STUMBLE
Make a DC 15 Dexterity Check
If the check fails, you fall down stunned for 3 rounds
1 - STUMBLE
Make a DC 15 Dexterity Check
If the check fails, you fall down stunned for 4 rounds
1 - STUMBLE
Make a DC 15 Dexterity Check
If the check fails, you fall down stunned for 5 rounds
1 - STUMBLE
Make a DC 15 Dexterity Check
If the check fails, you fall down stunned for 6 rounds
2 - TRIP
You fall down stunned for 1d6 rounds
4 - OFF BALANCE
Make a DC 10 Dexterity Check
If the check fails, you have no attack next round
4 - LOSE GRIP, DROP WEAPON
Make a DC 10 Dexterity Check to recover the weapon
2 - TANGLED
Roll 1d6 to see if you lose your attack next round:
1-2 Make a DC10 STR check to avoid
3-4 Make a DC15 STR check to avoid
5-6 can not avoid
2 - WEAPON KNOCKED AWAY
Roll 1d8 for direction and 1d10 (result in feet) for distance to where the weapon lands
2 - WEAPON BREAKS
100% base chance (-20% for each +1 the weapon has) that the weapon breaks
2 - HIT SELF
Roll 1d6 to determine amount of damage:
1-2 Half
3-4 Normal
5-6 Critical
2 - HIT FRIEND
Roll 1d6 to determine amount of damage:
1-2 Half
3-4 Normal
5-6 Critical
2 - TWIST ANKLE
Your speed is halved
Make a DC 10 Dexterity Check
If the check fails, you fall down prone
1 - EQUIPMENT INTERFERENCE
Make a DC 10 Dexterity Check to fix
-6 to all attack rolls until problem is fixed
1 - DRAW TWO MORE CARDS
2 - NOTHING HAPPENS
I still have the Word documents for these if anyone is interested.