I have never, ever played in a game with the rules as written. Not once. Every game, even board games, tend to be tweaked to fit the participant's wants and needs.
For example, I just played Risk GodStorm last weekend. If you've never played it, it involves conquering the ancient western world (including Atlantis) and gods, temples, and the underworld play a big part. There are 4 gods that every civilization can call on: War, Death, Air, and Magic, and they all have the same rules across the different civilizations. Every god of war is the same. After playing, even a bit during the game, we spent some time talking about tweaking the rules to give everyone slightly different rules so that the choice of civilization actually has an impact on the game.
In my old school D&D games, I use the following house rules.
"Order of the d30"**
Once per game session, a player may choose to roll a d30 instead of any normal dice roll. This cannot be used for any purpose during character creation or for hit point rolls.
"Chop When they Drop"
Fighters dedicate their entire lives to mastering their skill at arms. Because of this single-minded dedication, fighters (not including dwarves or elves) are able to dispatch multiple foes at a rate that astonishes other classes. Anytime a fighter kills an opponent, he immediately gains a free attack on any other enemy within reach. Should he slay the opponent as well, he gains another free attack on a nearby enemy. This series of events continues until the fighter either misses, fails to kill and opponent, or runs out of enemies within the reach of his weapon.
"Shields Shall Be Splintered"
Shields Shall Be Splintered: Anytime you are about to take damage and have a shield equipped, you may choose to attempt to sacrifice the shield in order to avoid incurring the wound. Make a saving throw vs. death and, if the save is successful, the shield is sundered by the blow and destroyed, but you take no damage. In the case of spells that allow a saving throw for half damage, you may invoke this rule if you fail your save against the spell. Doing so successfully reduces the damage by half. In the case of magical shields, invoking this rule successfully means that you take no damage from blows (or half from spells) but the shield loses one "plus" from its enchantment. Thus, a +1 shield would become a normal shield, a +2 shield becomes a +1 shield, etc.
Dutch Courage: Any character may heal themselves by spending twenty minutes binding their wounds, catching their breath, and consuming a pint of wine. Doing so heals the character of 1d6 points of damage. This method of first aid may only be used once per day.
When a character reaches zero hit points, he or she is falls unconscious. Players then roll on the Mortal Wounds chart from ACKS. This rule replaces the previous "unconsciousness and death" rule.
Magic users may use the Zap spells as outline in this blog post.
**This rule is also in effect for my 4e game.