We've rented from cyberrentals.com, vrbo.com and orlandovillas.com --
The orlandovillas.com website is I think the same as the UK one listed in the posting above. They have a section on "late availability" that we have gotten some great deals from. Also - a "Florida forums" discussion board that offers great discussion on the different neighborhoods etc. I have no vested interest in the site - we've just rented 5 different houses from there over the past 5 years and had a great experience every time.
A few questions we usually ask before renting a house - square footage (some houses are 4 bedroom but really, really small), size of pool and is pool heat included in price, privacy of pool and/or spa, is there a gameroom? (great for Disney downtime), is there a shortcut to Disney via backroads or will you need to take major highway? and if so what is the drivetime to Disney during traffic hours?