The Pell Grant is certainly not the only grant funded by the U.S. government. Anyone interested in attending college would be well advised to research the grants; there's a good chance of finding something that you qualify to receive.
The FAFSA site has a list of federal education grants, such as the Academic Competitiveness Grant and the National Science and Math Access to Retain Talent (SMART) Grant. There are also various loans and work study positions available. You can apply for all the federal funding sources using the FAFSA online application. The funds will be distributed in the same way as the Pell Grant, in most cases. Of course, there are many more possibilities for financial aid through state and local programs, so check every possible source for help.
Most government education grants begin when Congress sets aside money for individual grant programs. For example, $12,606,713,000 was appropriated for the Pell Grants for 2007. There's always a great deal of political pressure from various sides concerning the amount of money Congress allots to specific grant programs. You can learn more about the final appropriation amounts for other years for the Pell Grant and for various other grants at the U.S. Department of Education website.
The United States Department of Education (USDE) is in charge of the distribution of grants, loans, and work study agreements. USDE sends the money to schools and institutions, who pass the money to the students. However, if a student does not keep up his or her grades, the funds can be cancelled or cut off for future terms. So you must do your part by maintaining a qualifying GPA (check with your school for specific grant GPA requirements). Registrars are responsible for checking to see that students remain eligible for Pell Grants and other grants, and for distributing the funds to students.