I'm looking to buy a macro lense for my wife, who is an enthusiast/hobby photographer. Her camera body is a D7000, and she has a full suite of lenses other than a dedicated macro. I know nothing about photography, not even what an f stop is, so was wondering if someone could advise me on what I should look for, either a specific model, or specs in general. She'll probably shoot flowers and plants, along with small reptiles and amphibians around the garden (she hates bugs). My budget is about $500 Cdn (so currently under $9.99 US 😀). Seriously, about $400 US max.
This sounds like a product recommendation which I think is off topic. If I were you I would look into some auto focus extension tubes. I have a set for around 50$ that I through on a telephoto lens that does a great job. You could even buy a couple of sets and stack them to get some pretty high magnification.
At US$400, your options are pretty limited.
The Nikon 40mm macro lens is the cheapest option (about $275) and from all reports it's a solid lens. It'll autofocus and meter with the D7000 and any other modern Nikon body. But... you have to get thisclose to your subject to use it. So close that you are practically touching the subject. The minimum focus distance is 6.4" from the sensor plane so that's about two inches from the front of the lens to take a photo of something at a 1:1 aspect ratio. There's literally no room to light your subject.
The Tokina 100mm macro lens is right at your budget ($400). It's an older lens but an advantage of the D7000 is that it will let you use autofocus with this kind of lens. Working distance will be good and results will be pretty good. All around, a solid buy.
The Nikon 55mm macro lens is right at your budget ($400). A very old lens, your wife will need to manually focus and meter with the lens. It only focuses down to half life size, a 1:2 ratio, but that's often still plenty to take photos of things like flowers. A great lens, if you know what you're getting in to.
Push the budget up to $500 and several really good options become available. Nikon's older 60mm, Nikon's 85mm, Tamron's 90mm, and Sigma's 70mm.
Another direction, as rob describes, is some extension tubes. You won't get near as close as with a true macro lens, but maybe that's not necessary. Reversing rings might also work well, if you've got a slew of lenses to work with.
The main things to consider when researching what macro lens to buy, are the magnification of the lens, and what the focal length will yield as a working distance and whether that working distance is appropriate for the subject matter you got the lens to shoot.
A number of 3rd-party lenses are labelled as 'macro' (or in Nikon terminology MICRO) but that magnify at less than 1:1. When a lens's magnification spec is given as a ratio, that ratio is the size of the image on the sensor to the size of the actual object. 1:1, means that with an APS-C sensor, you can fill the frame with a subject that's the same size as the sensor: in the case of the D7000, that means 23.6mm x 15.6mm. Most folks consider 1:1 magnification the requirement for a "true" macro lens. All "true" macro lenses are prime lenses--lenses that don't zoom.
The other factor to consider is how close you have to be to your subject while shooting macro. The shorter the lens, the smaller this distance tends to be. With flowers or table-top arrangements, or product photography, this may not matter. With subjects that can crawl, hop, or fly away if you loom too closely over them, it may. So, for example, a 60mm lens is probably better for still life photos, while a 90mm or 100mm macro might be better for bugs.
In addition to macro lenses, you can also increase magnification with "poor man's macro" methods, such as reversal rings (putting a lens on the camera backwards, or attached backwards to another lens with a coupling ring where the filters go), close-up filters (like putting a magnifying glass in front of a lens), or extension tubes (which increase the space between the camera body and the lens). But all three of these techniques remove the lens's ability to focus past a certain distance, and will focus strictly by adjusting the lens-to-subject distance, so you don't get to choose your framing as freely as a macro lens lets you do. And all of these techniques also can work in conjunction with a macro lens.