title_body: Coffee stain in coffee cups
upvoted_answer: Baking soda. I use a standard dish sponge/scrubbie with baking soda and a little water and it comes completely out. I've actually tried it with just a damp paper towel and baking soda and it's almost as easy.
  
  Baking soda has natural whitening properties and is a very gentle abrasive. I actually started using it elsewhere (sinks, counter tops, greasy pans) before discovering its use in coffee cups.
  
  I've also tried baking soda + salt on counter tops and pans with great success. Salt has similar whitening properties (surprisingly to me) and is a stronger abrasive. Haven't tried it in coffee cups, but you might consider trying that if you need a little extra cleaning power. (Also: Aside from being cheap, baking soda / salt are food-grade and non-toxic!)