LIQUITEX! Excellent. All my paint additives are Liquitex. I think I have everything they make on my desk. But I'll give you a run down of what I have and uses.From DakkaDakka.
Matte Medium - Use in all my paint mixes 1:1:1 water/medium/paint. Helps flow and keeps the pigment from separating longer than just using water.
Flow Aid - Use when I layer blend. Drying retarder and helps to get great coverage with heavy dilution.
Slow Dry - Use when wet blending. 1:1:1:1 water/medium/color/slow dry is how I mix it. Drying retarder of coarse.
Glazing Medium - Makes awesome glaze mixes. Dries fast for fast layering. Formulated to cover well and level to the surface. Glaze vs Wash = Glaze made to evenly cover and Wash made to pool in recesses.