PepsiCo and Coca Cola control about 75% of the carbonated beverages market in the US. Dr Pepper Snapple Group (a separate company entirely, Pepsi doesn't make a Dr-type soda) has about 17%. The rest is a whole mass of specialty and local producers.

Interestingly, you will find Dr Pepper in most quick service restaurants despite their exclusive contracts. Those contracts are not with a particular brand, but with a particular distributor. Dr Pepper has cross-distribution contracts with a bunch of major distributors so they get to ride along.