Most human shampoos can safely be used on horses bodies, tails, and manes. A human ph level is the same as a horse so most "normal" human shampoo can be used on them. Many horse owners actually prefer people shampoo, it's usually much more affordable than commerical grooming products and because humans can provide feedback on a product, most horse shampoos rinse cleaner with less residue than shampoos marketed specifically for horses.
Dish soap also works extremely well on lighter colored horses. Soap made specifically for hand washing dishes is powerful, but specially formulated to remove dirt without harming the natural oils of the skin- perfect for a horse shampoo!
Below are some of the shampoos frequently used on horses with excellent results:
Link products: http://www.drugstore.com/products/prod.asp?pid=152322&catid=34547&trx=PLST-0-SEARCH&trxp1=34547&trxp2=152322&trxp3=1&trxp4=0&btrx=BUY-PLST-0-SEARCH