I wrote the original post on this subject in 2007 and not surprisingly, that post has been the most popular subject on the PFRE blog. This is because a wide-angle lens is something everyone getting into real estate must get before anything else because you can't get by shooting interiors with off-the-shelf kit lenses that come with cameras!
When talking about wide-angle lenses, it's important to understand that the sensor size of the camera the lens is mounted on has an effect on the angle of view of the lens:
A practical example of this is if you put a lens like a 12mm Rokinon on a Sony A6000, you must multiply the 12mm by the Sony cropped sensor multiplier (1.5) to get the effective focal length (18mm). So we say the effective focal length of the 12mm Rokinon on a Sony A6000 is 18mm.
This focal length multiplier is a big deal for real estate photographers because this means the standard kit lens that comes with most off-the-shelf cameras is not optimal for real estate work because after you multiply the effective focal length multiplier for the camera you are using, the lens may not have as wide a view as you think.
Real estate photographers find the effective focal lengths roughly between 16mm and 24mm to be the "sweet spot" for shooting interiors. It's best to have a zoom that covers this whole range between 16 and 24 but at a minimum, you need to work at 24mm or below. This is why the Sigma 10-20mm lens (available for both Nikon and Canon) is so popular with real estate photographers because with a 1.6 multiplier, it allows you to work between 16 and 32mm effective focal length and it's an inexpensive alternative. For full-frame cameras, the Canon 17-40mm, Canon 16-35mm, and Nikon 17-35mm lenses are popular choices for interiors.
What are the wide-angle alternatives for APS cameras?
There are many wide-angle prime lenses that will work but for real estate, a zoom that covers the range of 16mm to 24mm, 35mm effective focal length will serve you best.
The bottom line here is that you need to pay careful attention to which lens you choose for real estate work. It may be the most important equipment decision you make for real estate photography. It's way more important than which camera body you choose.