Then normally, for most people, you would require a visa. However, keep reading, there is good news, I'm just including all information for future people reading this.
VisaHQ - for Indian passport, living in the US, you still need a visa, whether for business, tourist or transit purposes.
Citizenship and Immigration Canada also confirms - visitors to Canada from India require a visa.
The Niagara Falls Live Travel Tips page also confirms:
Persons wishing to travel to Canada whom are not U.S. citizens (ie:
foreigner) may be required to get a tourist visa from the Canadian
consulate in the country from which they are arriving from. If you are
in the USA on a visa and wish to visit Canada while in New York there
is a Canadian consulate located in Buffalo New York. YOU CANNOT OBTAIN
A TOURIST VISA AT ANY OF THE BORDER CROSSINGS. When contacting the
Consul, inform them that you wish to obtain a "temporary resident
visa". As of April 1st 2012 the fee per person is $75 for a single
entry visa, $150 for a multiple entry visa or $400 for a family
(multiple or single entry).
However, going back to the Canadian Citizenship and Immigration page:
Visitor Visa Exemptions
Many people do not require a visa to visit Canada. These include:
- persons lawfully admitted to the United States for permanent residence who are in possession of their alien registration card
(Green card) or can provide other evidence of permanent residence;
so it appears, in fact, that you would be totally fine, as you are in possession of a valid green card!
Update - As of 2016, there is a new entry requirement in effect. US Permanent Residents ("Green Card" holders) entering or transiting Canada by air must be approved for an eTA (Electronic Travel Authorization) to fly to or transit through Canada unless they are otherwise exempt. This requirement does not apply to those entering by land or sea.