I once did a system that used d20 (it started out D% but we decided it had too much chance of failure.
Anyway the system was rules light, skill-based and generally freeform
Ability Scores used a 6 points and ranged from -3 to +3 (with modifiers for race etc) eg a Giant might have a Power+3 thus gaining a Max Power of +6
The Abilities were
POWER (Str/Con), AGILITY (Balance and Flexibility), DEXTERITY (Hand-eye Coordination), PERCEPTION (Observation, Senses), INTELLIGENCE (Knowledge and Learning), WISDOM (Arcane skills, Insight, Sense Motive etc), CHARISMA (Presence and Diplomacy etc)
Anyway these abilities were the basis of Proficiency Tests and a PC could develop a proficiency in anything they wanted (from Abseiling to Zoology

)
Anyway each character got a PDP (Proficency Development Pool) of d10 points and additional PDPs basically replaced XP in the system
so for instance a PC with high POWER might take Proficiencies in Lifting Weights (+2 to Power), Endurance (+1) and Fortitude (+2)
When ever they were called on to do a task they would then test their Aptitude (Ability+Proficiency) against a set Difficulty using d20+Aptitude
eg Malcom the Barbarian (POW +2 Lifting Weights +2 Endurance +1 and Fortitude +2) needs to Move the giant boulder that is blocking the entrance to the Cave mouth. He argues that his lefting weights proficincy should help and so does the test of d20+Pow+ Lifting Weights (D20+2+2) against a Difficulty of 18
if Malcolm had a 'Push' proficiency the Difficulty might have been 15 instead
COMBAT
Weapon proficiencies were based on Coordination and each Weapon was a seperate proficiency
Attacks were made and then the target got a Defensive Proficincy test (Coordination for Parry, Agility for Dodges and Power for Damage Reduction) if they so chose
Armour added to Hit Points which was POW+D10
POWERS (Feats and Class abilities)
POWERS were also Proficincies but usually had a higher PDP cost. So for instance Malcom might chose the Rage Power with a PD cost 3 and this would be added as a new POWER Proficiency. Spellcasting was also a Proficiency (based on WISDOM) with each School being a seperate proficiency (Spells were created freeform using a particular formula based on School (determined effect), Target size, range, duration, and the power and complexity of the spell.)