The most popular subject on this blog is wide-angle lenses. The most popular post on the PFRE is the 2007 post on wide-angle lenses. It gets a staggering amount of traffic, day in and day out. Makes sense I guess, wide-angle glass is the most important piece of gear a real estate photographer has.
I've done posts on the most popular lens in real estate photography (the Canon 10-22mm). But when recommending wide-angle lenses to beginners besides the Canon 10-22mm I always mention the Sigma 10-20mm. It is a couple hundred dollars less than the Canon but is available for Canon, Nikon, Sony, Samsung, Pentax, Minolta and Sigma DSLRs. According to my lens poll on the lens page, it is the second most popular wide-angle lens used in real estate photography.
While some photographers report that they have upgraded to the Canon 10-22 and like it better, many others report they love the Sigma 10-20mm and would not use anything else. The Sigma 10-20mm has it's own flickr group with 15,000+ members and 285,000 photos! The Canon 10-22 has a flickr group too but it's not as big.
So if you are using a DSLR with an APC-C sensor (not full frame) and you are on a budget you should consider the Sigma 10-20mm. You don't have to buy it new either. Right now on Amazon there are 19 used Sigma 10-20mm lenses priced between $434 USD and $360 USD.