As Boyink mentions, MLS listing is where the problem lays - Back in the late 90s I did the first real estate site (national) that had MSL integration and mapping (the forerunner to Trulia, Realtor.com, Zillow, etc). This was for Moore Data, at the time the largest manager of MLS data (the EDS of real estate). The actual MLS data is specific to each individual real estate organization and is far from standardized. In my area (Mpls) it’s controlled by one association but in some (Chicago I recall) it was multiple associations all with different data structures. So, it’s very hard to develop a general plugin or module.
That said, it depends on exactly what you want to do…
If you want to pull listings into an EE channel is one thing.
If you just need listings integrated into a page/section, there are many services that can provide iFrames, widgets, etc. Search IDX in Google.
Some of the packages out there (Open Realty) are CMSs specifically for real estate and include mail list, crm, etc.
So, as I mentioned, it depends on “exactly” what you need to do.
Am interested in any and all EE integrated solutions, or what others are doing in this area - Get asked about this all the time.