Many major companies that you have all heard of, Colgate, Crest, etc, are now selling whitening products that you can easily purchase at your local pharmacy or grocery store.
These include non-custom trays with whitening gel, whitening strips, or whitening pastes that can be applied to the teeth in accordance with the instructions in order to whiten.
Because of the wide variety of products in this category, it is impossible to accurately review them all. But in general, these products are a large step below in-office and custom tray whitening. Usually the levels of whitening gel are significantly decreased, tooth contact is decreased, and overall effectiveness is thus diminished.
That is not to say that you should never use these products. In the case of a quick touch up, nothing major, just to get your teeth to that sparkling white again after one too man cups of coffee that day, these products may be exactly what you need.
However in the case where your teeth have been tarnished by years of wear and tear, coffee, wine, and other stainers, in office teeth whitening or long-term custom tray whitening are certainly superior to these over-the-counter products.