Both
www.realtor.com and realestate.yahoo.com have good features. Other sites that are good as reference:
www.zillow.com - gives a "zestimate" of value. Some home owners have "claimed" their home and have additional information - very rough info, but it is a very well layed out site.
www.dcad.org - for dallas county, tax roles - so you can see the tax rates for different taxing authorities (schools, city, county, etc)
www.collincad.org - same as dcad.org, but for collin county
www.tad.org - same ad dcad... but for tarrant county
I haven't used collincad or tad.org as much as dcad.org - but DCAD has an interesting map function that you can map out 100-year flood plains, school district borders, trails and such. Very nice resource.
There are of course city web sites also, and many of the city sites include maps to the trails within those cities, which are nice to see. They also list city resources such as Allen's Natatorium - wonderful price and amenities... I used it as a gym when I lived up there.