Golden Acrylic paints - They're super pigment rich and beautiful to work with. I use the Golden Fluids a lot because you can use them as "watercolors" or "inks" by adding water to them and I love the movement you can achieve out of them.
Van Dyke Brown and Prussian Blue paint (or colors with similar value and hue) - I often use these to mix my own "black" which has more depth and light reflection than actual black
Quinacridone Magenta paint - I like to add it to other colors for a bit of pop or warmth and I love that it can be used with warm or cool colors and not get "muddy" (most of the time ;)
Nova Color Paint - These are also pigment rich and significantly less expensive than Golden paints, and come in all variations of viscosity
Sennelier Oil Pastels are great for adding line and color to paintings - especially when I'm stuck on a piece and want to add pops of color or texture to an acrylic or multi media painting
Old hotel cards - I collect these for use as a palette knife to apply large quantities of thick paint to paintings to add texture to pieces, to cover up the uglies or to add crisp lines to a painting by dipping the edge in fluid paint and pressing the edge onto the painting
Skewer or a sharp pencil for etching out fine lines in thick paint
Paper Towel Roll - When I'm working on a large piece I use this to drag and blend paint across the canvas, similar to the way a palette knife is used to spread paint around and achieve beautifully blended textures
Baby wipes are fantastic for adding and removing paint to the canvas. I also dip my brushes straight into the package, swirl it around for a quick clean of the brush when I'm working quickly and don't want to use my water container for cleaning my brushes
Foam brushes are my favorite tool for painting the edges of paintings when I'm finished because they come in a variety of sizes and are easier to control so as not to get unwanted paint on the finished piece
I hope you've found one or two goodies to add to your toolbox from the list.