I cooked up this Knack system some time ago, and have been using it to good effect in my ongoing OSE campaign. It provides a simple approach to skills while adding flavor and customization to character backgrounds and level progression.
Character Knacks are a modular sub-system for B/X D&D (and various retroclones), and fills the niche that skills might take in other games (also compatible with my Character Boons subsystem). The purpose of Knacks is to provide more varied options when leveling up, to give the character a small numerical bump with related tasks, and to speak a little to the character's personality, background and interests. Although the utility and power of a Knack are somewhat limited and specific, I tend to give my players broad ground to argue for their application.
A Knack is a minor skill, hobby, or area of knowledge. They are always narrow in scope, non-combat oriented, and can’t take the place, or reproduce another class ability. If your Knack can be plausibly applied to a (non-combat) related ability check (or x in 6 test), the GM may award a small bonus to the dice roll (typically +1, +2, or advantage).
Knacks can be created by the player, selected from the big list below, or rolled randomly on a d100. If the GM agrees, a Knack may be taken twice, granting superior expertise in the field. Each character starts with 1 Knack, and gains another every two levels (2, 4, 6, 8, 10, 12, 14, etc).