I generally make my own washes, using either oils and mineral spirits, or craft store acrylics and water. As Don noted, you need them very thin, ie, heavy on the thinner and very light on the color.
I have also bought two of AK Interactive's prepared washes, just to see what all the fuss is about. They are very easy to use, since they are premixed and have stabilizers and other compounds to take the guesswork out of making a wash. I can see how modelers would want to use them. For me, it's a point of pride to do it the old (hard) way and experiment and learn to make my own.
Also, I tend to use either black, or to use dark browns, burnt siena or burnt umber, for nearly all of my washes. I found that the browns especially go well with a wide range of colors. That is to say, when I compare a recess with a black wash to one with a brown wash, the brown wash more often looks like the natural shadow or shade that you'd see, under natural light, than the black does. I also use the brown washes on pilot and ground crew figures in 1/48 or smaller, as a quick and easy way to add shading, to faces, uniforms and gear.