Buses are probably the most cost-effective way of travelling in the USA. Greyhound buses are a long-standing and well known national company and travel to most of the major cities, with air conditioning and sometimes even WiFi, but there are hundreds of other great companies that run a more regional service.
Despite the cheap cost of buses, America is a big place and they can often be slow, so if you’re up on cash and down on time, consider another option.