1. Added effect+poison, a chance to poison creature with the use of combined weapon.
2. Added effect+time, a chance to slow or stop creature attacked with paired weapon.
You  can also connect Added effect to almost any other materia but I found these most useful.

Here are a few more complex materia combos for you.

1. Mega-all,Deathblow-hp absorb-  Restores the first 3 numbers of the amount of points taken out of the enemy.
2. Mega-all,Mug-hp absorb-  Hurts and steals from all opponents while restoring a small amount of HP.
3. Cover, Counter Attack,Counter Attack, Command-counter-Deathblow-  Covers for all other allies and counter attacks 3 times finishing with deathblow.(this is my favorite materia combo because you can attack several times in one turn)
4. Sneak attack-deathblow, Mega-all- You will start the battle by using Deathblow on all opponents.
5. Sneak attack-mug, Mega-all- You will start the battle by mugging all opponents.