Fortune 500 is a list of largest US companies by total revenue. It includes familiar household names such as Apple or Ford, but also more obscure companies such as Berkshire Hathaway. In 2014, the top of the list is held by Wal-Mart with revenues of 476 billion dollars, while United Rentals on 500th place has annual revenue of 5 billion. The total revenue of all 500 companies on Fortune list is 12 trillion dollars. Just to put the things in perspective - the GDP of USA is 15 trillion dollars and the estimate of total world GDP is 72 billion dollars. So, if you want to know where the money is look no further than this list of 500 most successful American companies.
When selling enterprise software the holy grail of success is getting Fortune 500 companies on-board. While you might start making software for smaller enterprises, the end goal of every enterprise software company must be the 500 biggest companies which together represent 75% of total market. Similarly, when people talk about selling to top brands, they mean selling to Fortune 500 companies as the list of top brands and list of top companies by revenue match almost exactly. Fortune 500 companies are also the reason why enterprise software sucks (and will always suck) for end users. Even if the software vendor starts with a small, elegant solution it will end up with complex monster system to accommodate all the whims and wishes needed to close a deal with a Fortune 500 company.