www.realtor.com it shows about the same mls as the realtors use.
I also like www.unitedcountry.com just because it allows you to search by the whole country, a region, state, or their office locations. I can input I want 80 acres with 3 bedrooms for under $200k and find one somewhere in this country.
If realtor.com would change their interface from being flash based with all kinds of moving pictures and ads it would be a huge improvement. I now only use it for daily updates. I wish realtor.com would also allow a search by state or other area beyond just a city area.