These are house rules I have for my B/X games, and most of these rules are taken directly from rjbs’ list at http://dudgeonmaster.org/games/beyond/house-rules.html.
Max Hit Points
All level 1 and level 2 character start with maximum hitpoints.
Ordinary People in Extraordinary Circumstances
Character statistics are rolled 3d6 in order. The human classes do not require any stat minimum to qualify, so you can play a stupid Wizard, foolish Cleric, klutzy Thief, or puny Fighter in the dice rolls go that way. If you are unhappy with your character to the point of considering kamikaze attacks, at least get killed as smartly as possible by absorbing some hits that might land on better-qualified adventurers.
Every character receives one Awesome Point per game day. Awesome Points can be spent to replace any die or dice roll with a d30.
I use Jeff’s rules for carousing. In brief, characters can spend a lot of gold on debauchery and convert it into experience points. This runs the risk of drunken mishaps, however, so drink responsibly. The minimum investment in this sort of revelry is 100 gold sovereigns.
Critical Hits & Fumbles
A natural 20 on the ‘to-hit’ roll indicates a critical strike and does maximum damage. A natural 1 indicates a fumble, and something bad happens.
Damage by Class
The damage your weapons do is mostly determined by your class, not your weapon. Fighters usually do 1d8 damage. Wizards do 1d4. Everybody else does 1d6. Some weapons stink and do one die worse, and some are brutal and do one better. Pick your weapon because of its awesomeness, not its damage. This rule is mostly a harkening back to original D&D rules.
Healing and Hit Dice
When the adventure is over and the characters get at least a full day and night of rest in a safe place, they reroll all their hit dice for healing. If the roll is lower than their current hit point total, it is unchanged. Until the adventure ends, characters can only regain one hit point from each full night’s rest. A full night’s rest in town doesn’t mean the adventure is over. The character needs a whole day off.
One Last Breath
Any time a PC runs out of hit points that character is allowed a saving throw versus Death if they haven’t already failed a saving throw to get 0 hp. If the roll is made the character is unconscious and at 1 hit point. Without aid from an ally, the character will die in 1d4 rounds.
Shields Shall Be Splintered!
This rule comes from Trollsmyth. With this house rule, you get the usual -1 to your AC with a shield. However, any time you take damage, you can opt instead to say your shield absorbed the force of the blow. The shield is shattered and must be discarded, but you don’t take any damage from that hit.
Stunts and Called Shots
If you want to do something totally awesome, you can shift down the threshold for a critical hit by shifting up the threshold for critical failures. In other words, you can say that you’ll crit on an 18-20, but then you’ll fumble on a 1-3.
Clerics: Simplified Turning of Undead
Thieves: 2d6 Thievery
I use Dyson Logos’ rules for 2d6 thievery. Basically, all those annoying percentile roles for thief skills are replaced with 2d6 checks.
Wizards and Elves: Casting from the Book
A wizard or elf can cast any spell that he has copied into his spellbook, without memorizing it. It takes six turns (one hour).
Wizards and Elves: Once More, with Feeling
A wizard or elf can re-cast any spell that she memorized for the day, at the cost of one hit point per spell level.
Wizards and Elves: Spellbooks
Wizards and elves begin with a spellbook with (3 + Int bonus) spells written in it. One of these is always Read Magic.