What are the top 5 monopolistic companies?

The 2008 Fortune 500 ranked the 5 largest companies, in terms of the Revenues (not profits), as follows:

1. Wal-Mart Stores

2. Exxon Mobil

3. Chevron

4. General Motors

5. ConocoPhillips Source: http://money.CNN.com/magazines/fortune/fortune500/2008/full_list/index.html