Downtown Alliance offers a free walking tour of Wall Street, two days a week.
From their web site:

Don’t miss this FREE 90-minute guided walking tour, weaving together history, events, architecture and people of Downtown - the birthplace of New York, the financial capital of the world, and the hottest new neighborhood in the city.

Stops include the U.S. Custom House, Trinity Church, Wall Street, the New York Stock Exchange and other architectural and cultural sites.

Date: Every Thursday and Saturday, rain or shine

Time: Noon

Place: Meet at the steps of the US Custom House at One Bowling Green

Cost: Free

No reservations necessary for individuals. No groups, please. For more information please call (212) 606-4064.

How to get there: 4,5 to Bowling Green | 1, 9 to Rector/Greenwich St. | N, R to Whitehall St. Google map.