Realestate.com.au
I just discovered that realestate.com.au is running a mobile version of their website (as a very trendy beta), which is actually extremely useful. It's obviously been designed with iPhone users in mind but it was fully functional on my Nokia E71.
Realestate.com.au is still has a few things missing, such as allowing for more fine-grained searches and displaying a bit more information about each property (such as what type it is). Overall though the site is easy to use and navigate. It's good to see that some large sites are starting to transition to functional mobile versions as it makes browsing the web from your phone a whole lot nicer.