Pearlescent Paint - Pearlescent paint can be found in most art stores, usually near the metallic paints. Whats different is that rather than use it as just another paint, by using just a small amount, you can glaze over almost any other paint. This allows the color to still be seen beneath and creates a depth to the surface.
Styrofoam - Styrofoam has many more uses than people realize. It can make a darned good canvas substitute. Using a flat piece allows you to dig or melt into the surface, creating interesting surfaces and depths. You can also melt or cut through creating an opening which can be back lit.
Fabric Softener - I use a small cap-full of fabric softener in several of my paint rinsing jars. The results are, softer and more pliable brushes and better smelling water that doesn't need to be changed quite so often. Keep a couple of jars available with clear water also for the final rinse before using the brush. Its also great for brushes that you thought were destroyed due to drying out by placing them in some non-diluted fabric softener for a day or two then gently brushing out the remaining crud.
Metal Cat Combs - I use a fine toothed metal cat comb to clean some of my brushes. These do well cleaning out fine particles of paint that may dry on the brush.
Fired/Glazed Tiles - I use fired/glazed tiles instead of paint trays to place wet paint. The advantages are, super easy cleanup with an exacto knife, easy to hold when working, easy for mixing colors. In most home improvement stores they will have a tile area where you can purchase a wide variety of ceramic tiles for bathrooms, etc. They will have individual tiles that are very cheap (about $1 each). I keep a stack of them in the studio in two sizes (3"x 6" and 6"x 6") The smaller size are easy to hold in the palm of the hand, while the larger is good for multiple colors. No more buying cheap pallets or having to clean expensive ones. An additional advantage is because the tile is fired, almost everything else cleans off them also. I use mine for mixing glues and epoxies and very rarely find that I cant remove the excess with a sharp exacto blade.
Lysol - Surprisingly Lysol is great for more than germs. It is a great substance for cleaning up wet or dry spills on your canvas. A bit of scrubbing or rubbing is usually enough to remove most if not all of the spill. I prefer the aerosol cans to the liquids but both will do the job. I usually spray it on my cleanup rag though rather than directly on the surface. Disadvantage is that it dries quickly, so you may need to reapply a couple times to the rag. But this is also an advantage because there is no drying time once clean.
Gorilla Glue - Gorilla glue (or equivalent product) is a foaming glue. If your unfamiliar with it, it goes on like a syrup but raises and foams to fill cracks when used as a glue. I use it also to create texture on paintings. It adheres perfectly to canvas or board and because it foams it can create some amazing patterns. Sometimes I use it straight out of the bottle as a drip or with a pallet knife to create patterns and move it where I want it. Keep in mind, once its on there, its there to stay! You do have a few minutes where you can remove it, but once the drying begins, its pretty much permanent. As long as you clean your pallet knife within 5 minutes or so, you can get it off without any trouble. Painting over it is not a problem, although an undercoat or double coat is sometimes needed to cover the pale yellow color of the glue itself.
Toilet Paper - Want an unusual way to create texture? Lay the toilet paper out on the canvas and spray it with a mister made up of water and glue or water and mod podge. Once it dries it will harden enough and adhere to the surface enough that you can then brush it or spray it with a heavier adhesive or glue. It creates some great crinkled patterns, or let the first layer dry then overlay it with more. If you lay it out in the opposing direction the crinkling effect will create layered striations. Another trick is to use a heavier towel paper first then overlay with the toilet paper if you wanted higher ridges. Its tricky getting the hang of it, but great for creating something unusual.
Paint Samples - At most home improvement stores, there is an area in the paint department where they put out cans of sample paints used for color matching. This paint is pretty much wasted if the color is off or the customer does not like the tint. For those artists who are on an extreme budget, this can be a great thing. I found early on while working on huge canvas sizes, that I could never afford the amount of gesso it takes to cure a canvas. Due to the nature of my work, the canvas is completely covered in the end anyway. For me its crucial that the canvas weave is tight. One day I decided out of curiosity to see what latex paint would do. I know that gesso has special agents within it that make it great, but I thought "what the heck" and bought a $2.00 quart can off the reject table at Home Depot and tried it. To my surprise it was great! It covered nicely, it tightened the weave and it served nicely as an under layer. Since then I grab up a couple cans of this when I am in store and keep them on hand. They seem to be a cheap and viable method for curing your canvas prior to beginning your work.
Feel free to share your tips. We'll add them to the list!