A lighttable is a very useful piece of equipment.
If you have a sketch and want to trace the outlines of it in a clean way (instead of drawing them directly on the paper with the sketch and erasing the pencil afterwards), just put the sketch on the lighttable with another sheet of paper on it and copy the outlines of the sketch to it. That way you will also keep the original sketch for the future.

A lighttable is also useful if you want to have an inverted look at your picture (just flip your paper to the back side). It is easier to recognize mistakes that way, especially proportions, perspective, etc. However, looking at your art in a mirror or holding your paper against a light source (e.g. window) will do that job just as well.

I used to have a large lighttable in my room, but because of the size I got myself a smaller portable one (see the picture below), which I got from an arts and crafts store in USA. It's great for letter sized artwork. Anything bigger than DIN A3 will become difficult to trace on it though.

Masking Fluid / Masking Tape

Sometimes a picture requires a part to be left blank (e.g. for coloring at the end), while you want to paint the background around. It can be very hard to paint "around" a character, so you can use masking fluid instead: a liquid (white or colorless) with rubber/latex pigmentation for masking out areas of work needing protection (when color is applied in broad washes). When it's dry, it feels like a thin layer of rubber. However, it shouldn't stay on the paper longer as neccessary (for best results remove within 24 hours). The brush must be cleaned immediately after use (otherwise your brush will be permanently damaged).

For work like airbrush, there is also a special masking tape that you can put on your picture and cut out the area to be protected (with a scalpel).

Artist Tape

There is a special artist tape (usually white), which almost feels like paper (it can be easily ripped by hand, yet it's pretty solid). It is smooth, acid free and repositionable (no traces or damages when removed). You can use this tape for holding your artwork to drawing boards (by putting a stripe of tape on all 4 paper edges), so it won't shift while painting. Also, when drawing with water colors, it will help keeping the paper in shape (so it won't ripple too much). Don't remove it while the paper is still wet (or the paper might tear).

Also the smooth, white surface is great for writing on, so you can use it for labeling purposes.