I put together a Hybrid D&D edition for myself this past summer with some house-rule additions/tweaks. I haven't fully playtested it, it was something I put together to run when feeling a bit nostalgic to run some D&D when not running my usual system. A quick rundown follows.
Stats are generated roll 4d6, reroll any 1's and keep the highest 3 results, generate six numbers and assign as desired. Using the 4E stat bonus/penalty ranges.
Race and Class are as in the old red Basic box set, using BECMI xp tables with a level 10 cap (demi-humans are not level restricted below 10th). Racial stat bonus/penalties tweaked slightly.
HD and HP are not determined by Race/Class but by size. Small creatures use a d6, medium a d8, large a d10, etc. Incorporated a Wound/Vitality system such that 1stand 2nd levels are Wounds, the more damaged sustained the greater all actions are penalized. All con bonus/penalties apply to wounds. Every level above 2nd level is Vitality, which are lost first, except for any critical hits (i.e natural 20) which apply directly to Wounds. Unconsciousness at 0 or lower HP, Death at -Con value.
Class/Race use the 4e skill system with each having 1 or 2 assigned trained skills and a choice of 3 or 4 more to be chosen as trained. Racial abilities tweaked to fit in as being trained under conditional circumstances.
Multi/Dual classing does not exist, came up with what I call Crossover Packages, each character gets to choose from at every 3rd level, granting different spell list (i.e. druidical or illussionist) and/or several more skills now considered to be trained.
Using weapons proficiencies from 1e/2e in combination with the 3x BAB and level dependency. Picking up a new weapon proficiency at 4th level for a fighter has a BAB of +1 with that weapon, not +4, which increases to a BAB of +2 at level 5. Weapons are broken up into categories: Simple 1 Handed, Simple, 2 Handed, Martial 1 Handed, Martial 2 Handed, Simple Ranged and Martial Ranged, doing 1d6, 1d8, 1d10, 1d12, 1d8 and 1d10 damage respectively, Class/Race will affect which weapon categories are available.
Saves are taken from 3e with assigned values at 1st level per 3e by class. Higher levels award 2 points to be assigned to whichever save the player chooses (or split between 2 different saves).
Using ascending AC from 3e, but tweaked the armors available and their value.
Magic is greatly tweaked. Casters are more less similiar to the 3x sorcerer, using a mana/faith point system (Clerics can't cast until 2nd level) using Int or Wis bonus/penalties as modifiers to the number of points available. A spells level dictates how many points it costs to cast the spell, points are regained upon 8 hours of rest and a 1 hour meditation/study period. All spells taken from the 1e PHB only. Clerics pray for the spell, Magic User's/Elves can only use spells recorded in their spell book (unless read from a scroll, using a device). To add a spell to a spellbook, Magic Users/Elves must use the "% to know spell" from the 1e Intelligience table, if they fail, they have to wait to gain a level to try again. All damage/healing is at 1d8 per caster level be it divine or arcane. Class/Race does affect what spell list a character knows (Elves get the MU and Druidical lists to choose from to start, all other classes must take a Crossover Package to open up anything but the basic class list).
For everything else, I try to stick with the BECMI rules for their simplicity.