Suppose I'm in Canada visiting the Niagara Falls. Now I want to walk over the Rainbow bridge to visit the American side of the falls. Do I need a visa for this? Or is there any special onsite visa for this purpose?
So I general: Do the same visa rules apply when walking over the Rainbow bridge as entering USA on a normal border?
The Rainbow Bridge is a land border crossing between Canada and the US. There are no facilities to cross it without departing one country and entering the other, so as a result you will need whatever legal status is required to enter the other country - which obviously depends on your citizenship.
If you are a Canadian or US citizen then you do not normally need a Visa, but you will require a passport or a passport card. There are exceptions to this if you have a criminal record, etc, but these are the same for any Canadian citizen entering the US or vice versa.
If you are a member of a Visa Waiver Program (VWP) for entering the US then you do not normally need a visa (again, with the usual exceptions around criminal records, etc). As you are entering by land you do not need an approved ETSA as you would if entering by air, however you will need to fill in an I94W form.
If you are from a country that requires a visa to enter the US, then you will require a visa. Technically if you do not have a visa then the US would most likely "deport" you to the country you arrived from (ie, back over the bridge to Canada) - do not think of this as an easy way to get around the requirement if you are just expecting to see the bridge. Doing this will give you a permanent mark against your US immigration record, and make it harder to enter the US in the future!
The main thing you need to keep in mind is that you will be departing from and then re-entering Canada, and thus if you require a visa for entering Canada you should make sure that it will allow you to re-enter. eg, if you are on a single entry visa for Canada, you will technically not be allowed re-enter after visiting the USA.
The same immigration rules apply when crossing Rainbow Bridge as at any other land crossing into the United States. If you are a citizen of a Visa Waiver Program country, you are eligible to enter without applying for an ESTA (which you would have to do if you were entering by air or sea). There are also a few other countries whose citizens do not have to apply for a visa when entering the United States, but if this is not the case, you would have to apply for a standard B2 tourist visa. Do note that as far as I know, visas must be applied for at a US embassy or consulate; you cannot apply for one at the border crossing.
