Can anyone get the NEXUS card, and how?

While you don't need to be a frequent traveler, you do need to have a motivation to go to the airport at least once while not traveling (to be interviewed and have your iris scan) and to spend $50. I think it justified itself the first time I used it. You skip the entire lineup for customs and immigration and instead spend a few moments at a kiosk. It's marvelous. Occasionally you are sent for random inspection, and if you're breaking any rules (and the Nexus rules are stricter than the regular border rules) you lose your card. Forever. Trust me, that motivates you to stick to the stricter rules.

When the program started, you had to be both a citizen and a resident of the US or Canada. I was told to bring things like Hydro bills to prove I lived here. In 2012 this was relaxed somewhat - now citizens of Canada and the US who live there (but may not have done so for 3 years recently) are eligible, as well as "permanent residents." You have to tell someone why you tend to cross the border. There's a background check and some general investigating of you in advance, plus the interview. All well worth it.

In the years since Nexus was introduced the benefits have increased. For example, YYZ (and perhaps other airports) has a Nexus security line which saves you 20-30 minutes of waiting and guarantees you won't have to go through the nude-o-scope (I think they are also less demanding about shoes and belts.) I was also automatically enrolled in TSA Pre because I have Nexus. The time savings really add up.

You have to be either: 1) US/Canadian citizen or both (3-year residency requirement is waived in this case). 2) US/Canadian permanent resident, and 3) Have lived in either country for atleast 3 years (e.g., if you are US Permanent Resident but have lived in Canada for more than 3 years, or vice versa, that counts). The 3 year residency requirement is so that they can run a proper background check on you.

There's no hard and fast rule that you have to travel atleast N number of times with this frequency to qualify. Once eligible based on above requirements, they will ask you during the interview why you need one, then just say because it's convenient. Once they approve it noone's gonna check how frequently you use it.

The rules are here: http://www.cbsa-asfc.gc.ca/prog/nexus/elig-admis-eng.html

You have to be a citizen OR resident of the U.S. OR Canada. That is, if you are a resident of the US or Canada you can have some other passport. You do have to live in the US or Canada, but the former rule that you had to live here for three years is gone. NEXUS is cheaper than Global Entry and gives you everything GE does. You have to visit an enrollment center, which are located both at Canadian airports and at land border crossings -- for example I got mine in Fort Erie ON, across the river from Buffalo. You can only get the iris scans you need to use the Canadian immigration machines at Canadian centers; if you get your NEXUS at a US center you are supposed to be able to drop into any Canadian center without an appointment and get the scans added.

Little known fact: you can get a NEXUS card without having a passport, so for most purposes it's better and cheaper than the US passport card.