Knit Two Together (k2tog) - the easiest decrease: it's just like it sounds
Slip, Slip, Knit (ssk) - usually paired with the knit two together, they slant in opposite directions!
Purl Two Together (p2tog) - the easiest decrease, but with purls!
Slip, Slip, Purl (ssp) - the reverse of the knit two together (weird, right?)
Knit Three Together (k3tog) - just like the knit two together
Slip, Slip, Slip, Purl (sssp) - (coming soon!) like the ssp but with an extra stitch in there!
Slip One, Knit One, Pass Slipped Stitch Over (sl1, k1, psso) (coming soon!) - another way of doing a slip, slip, knit
Slip One, Knit Two Together, Pass Slipped Stitch Over (sl1, k2tog, psso) (coming soon!) - also called a Center Double Decrease