Several airlines have a program called "Visit USA" (VUSA) which allows
foreign nationals or US citizens who reside abroad to purchase tickets
that have unlimited standby travel within the US during their stay.
The tickets must be purchased abroad (usually in conjunction with an
international roundtrip ticket) and residents of Canada and
Mexico are ineligible. (Some of the programs are also not available to
people living in the Carribean.)  Other airlines with this program include
United Airlines, Northwest, and Delta.  Delta's program is called
"Delta Pass".

The pass is actually a set of coupons, with each coupon being good for
a sector. You must purchase at least N coupons, where N depends on the
airline. For United, the minimum is three coupons. There may also be a
maximum number of coupons. On United the price is about $90/sector if
you purchase the minimum number of coupons, and falls to $60/sector if
you purchase 10 coupons.  There is also a two-tier pricing scheme
depending on whether the Visit USA pass is issued by the same airline
you used to travel to the US. The difference is about $15/sector.
Prices and programs may differ on other airlines. For example, Delta
Airpass gives 30 days unlimited travel for about $500 (60 days $800).

No rerouting or refunds are allowed. (Some airlines will refund a
completely unused pass -- ask when you buy it -- but none will refund
a Visit USA pass after the first flight segment has been flown.) You
must make confirmed reservations for the first sector.  There is a
charge for changing the date on the first sector flight. Travel must
start within 30 days and must be completed withing 120 days of entry
into the US (for travel on United; other airlines may have different
policies). Open jaw travel is permitted. You may be limited to one
trans-continental direct flight per Visit USA pass, depending on the
airline.

Council Travel has been known to sell Visit USA tickets on USAir
without any restrictions at all -- you don't have to be a foreign
resident or national, and you don't have to be a student. Many travel
agents outside the US don't check whether you reside outside the US,
so you can buy the Visit USA coupons even if you are a US citizen.
They won't, however, send the tickets overseas; you have to be
physically present to pick up the tickets.