Liquitex do some great mediums and additives for your paints.
Matte medium - Boosts paint transparency - good for making washes
gloss medium - like matt medium but glossy finish
Glazing medium - Boosts transparency much more than the above two, has ultra glossy finish.
Airbrush medium - reduces viscosity of paints, like water but is made of acrylic binder so will not cause the pigments to seperate like water does at extreme dilutions. Great for airbrushing with.
Slow dry - slows drying

good for hot climbates or wet blending.
Flow aid - reduces surface tension in water for increased thinning effects (it makes your water even thinner than water)
Gesso - Acrylic Primer, artist quality. enough said.
Irridescant medium - adds a light interference effect; where it can appear metallic/pearly from one angle and solid colour from another.
Varnishes - protective coats...gloss matte and satin .. seems self explanatory. (not the same as gloss and matte mediums)
Texture mediums - adds 3dimensional texture to paints, can be used alone and then painted over or mixed with colour... black lava beads looks cool

might be good for basing now I think about it.
Off the top of my head that covers all (or most) of the mediums available from most decent artist paint companies