Palette and Masking
You should be spending most of your money on the paint, brushes and the canvas or paper, in that order. However, these are a few inexpensive extras I've discovered that will save you a lot of time. I also recommend you read my post on tempting artist's supplies that you don't need.
Masking Tape
If you've been reading this blog for long, you'll know I love masking tape. It's great for watercolor or acrylic. People always say "I can't even draw a straight line" - well, neither can I, and it doesn't matter - if you have masking tape you will never have to draw or paint a straight line again.
Cheap solution for beginners: You can buy the fancy art-store kinds, but in my opinion, the regular hardware-store type is fine for most applications. If using masking tape on paper, make sure you read my post on masking tape for tips before you start.
Liquid Mask
Liquid mask is a paint-on masking medium that you can use to protect areas of your paper or canvas while you paint a surrounding area. I use it to make white sparkles on water by painting specks of it on the blank paper, letting it dry, painting in the water, then peeling off the mask with my fingers, leaving tiny white areas of reflection. I also use it frequently to mask off the edges of a portrait subject so I can paint in the background behind.
Cheap Solution for beginners: Rubber cement is a bit harder to apply, but will work just as well. Use the handle end of your brush to apply it, as it is just about impossible to clean out of the brush hair.
Palettes
The palette is the surface that you use for mixing your paints while you work. There are all sorts of fancy palettes available. The dry watercolor tray sets usually come with a small palette built in. If you want a larger palette, you can choose from plastic, metal or porcelain. I would choose plastic or porcelain over metal because the white surface makes it easier to see what you're blending. Porcelain and plastic are both easy to clean with water. Plastic tends to stain a bit over repeated uses but it won't affect your ability to mix paints.
Cheap solution for beginners: You can buy specialized tray palettes for watercolor, but in my own experience I've found that a plain old dinner plate actually works just as well or better, and cleans easily with water.
Acrylic paint is actually pretty hard to get off of glass and ceramic palettes once it's dried. If you are using acrylic, disposable palette paper is the way to go. It's a water-resistant paper that comes in a pad. You use the pad as your palette (some even come with a thumb hole) and when you are finished with your day's painting, you just tear off the top sheet and throw it away. Based on the amount of water needed to clean a non-disposable palette, I think the disposable palettes are actually better for the environment.
Cheap solution for beginners: If you used up all your money on the paints and brushes, glossy magazines and flyers are a decent substitute for palette paper.
Small Spray Bottle
Keep your paint mixes from drying out by occasionally misting your palette with water while you're working.