Click to go to Google Maps satellite view
This airport has one official observation area, located on Lee Rd. across the street from end approach end of runway 27. It offers great views of landing traffic but you’ll be facing south and east so photography may not be too good at times other than late afternoons during the spring and summer.
The other spot that is popular but requires a parking fee is the roof level of the terminal A/B parking garage. From here, you can look at either the north side ramp or the south side so there are good opportunities for photography. from what I remember, the view is not as good from the roof of the terminal C/D garage which is where the international flights depart from.
Most traffic at IAH is Continental with their 737s, 757s, and Continental Express jets but there is also a good mix of international and other flights.
Click for photos from the Lee Rd. Observation Area
Click for photos from the A/B Parking Garage