What are some over-the-counter pills you can take for chronic back pain due to knotted muscles?

NSAIDS. Nonsteroidal anti-inflammatory drugs (NSAIDS) can be excellent for this type of pain. They can help with both pain relief and in reducing inflammation. Examples are ibuprofen (like Advil, Nuprin or Motrin), Naproxen sodium (like Aleve, (naproxen) Naprosyn, Anaprox) or Asprin (such as Bayer, Ecotrin, Ascriptin). Avoid NSAID's if pregnant or allergic to them.
Aleve, (naproxen) advil. There are no muscle relaxors over the counter. If your problem is knotted muscles, massage, self-massage with tennis balls, stretching, hot shower/contrast therapy with alternating heat/ice may all be helpful. Be sure, of course, there is nothing more serious going on by seeing a doctor. Relief with massage would strongly suggest it's muscular.