What are some effective over-the-counter teeth whiteners?

Dental. Speak to your dentist. Nothing over the counter will be anywhere near as effective as what your doctor can offer. Also, what you pay up front for the excellent benefits of teeth whitening for the dentist will be less, in the long run, than what you will pay for the otc stuff that falsely claim to whiten teeth. See the dentist!
White strips. Otc whitening strips are about the best. For better results get some custom made bleaching trays from your dentist. I do not recommend whitening toothpaste, gums, or mouthwash as i think they are mostly ineffective.