I compiled a list of skill affixes for better readability. All of them are datamined, so they may be inaccurate. I only included the highest level affixes on the list.
All of the skill bonuses are suffixes, so only one can spawn on a Magic item. Rare items are a question mark in this regard, because we don't know enough about the item generator. It's possible, that the number of skill affixes is limited to one even on Rare items.
These affixes start appearing on class-specific items in Nightmare and on generic items in Hell. What's interesting is that some of the highest level affixes can't spawn on the most powerful class-specific items, because their item level isn't high enough, but they can spawn on the generic items with the highest level. I hope this will change.
Code:Item Group 1: Quivers, Shields Item Group 2: Chest Armor, Cloaks Skill Bonus May Spawn on Item Group Bola Shot +5-14% Inreased Damage 1 Chakram Hatred Cost Reduced by 1-5 2 Cluster Arrow Hatred Cost Reduced by 1-5 1 Elemental Arrow +5-14% Inreased Damage 1 Entangling Shot +5-14% Inreased Damage 1 Evasive Fire +5-14% Inreased Damage 2 Grenades +5-14% Inreased Damage 2 Hungering Arrow +5-14% Inreased Damage 1 Impale Hatred Cost Reduced by 1-5 2 Multishot +2-8% Critical Chance 1 Rapid Fire +2-8% Critical Chance 1 Spiek Trap +5-14% Inreased Damage 2 Strafe +2-8% Critical Chance 2