They are also environmentally friendly, in that they can be tossed in with the laundry and used pretty much for at least a year or more.
Here is my favorite cotton yarn at usually $1.99 per roll:
Bernat Sugar and Cream All colors, from solids to ombres. Work semi-tightly for best result. I've made 3-4 washcloths/2 dishtowels per skein.
Nubby Scrubbies. Crochet. Awesome, easy, useful and looks great. Also a good pattern for cap edging I've discovered.
Wavey Dishcloth Knitted. Nice texture.
Waffle Knit Dishcloth. Knitted. Very popular on a nice site.
Swiffer Biffer Crochet/PDF FILE. *Awesome* money saving device for a Swiffer mop owner who can no longer afford the wet naps that go with it. Better for the earth too.
The washcloths can also be made in baby sizes, to put with layette sets if you are ambitious. I'm currently making a few to go with my booties and caps.