What are some of the best OTC remedies for ibs?

Fiber. Oddly enough, sometimes even diarrhea responds well to fiber which can act as a "bulking" agent and give stool more consistency and form. Metamucil is one name brand, and psyillium husks are also available, although there are plenty of generics. Constipation symptoms can be treated with fiber as well as laxative- such as miralax (polyethylene glycol). Pro-biotics may be of benefit and are also available otc.
Peppermint oil,flax& Agree peppermint oil is good- as enteric-coated capsules 0.2-0.4 ml 2-3x/day. Agree fiber & probiotics are good; flaxseed powder has been shown to be better than psyllium for ibs. Melatonin 2-5 mg may help. Agree with dr. Franz's comments re: diet under dr. Bina's answers; gluten, dairy and other food intolerances may be a factor. See http://www.Umm.Edu/altmed/articles/irritable-bowel-000098.Htm.
Butyric acid. Butyric acid heals large intestine available as butyric cal mag from biotics. Kapparrest which has curcumin and bioperin is anti-inflammatory takes away bowel irritation- need 10 caps bid, glutaime for IBS need 15gm per day.