Buildings are a great addition to any table. The background of the game you are playing will play a large roll in dictating the size, style and number of buildings you will want.
Many years ago, I got a few Armorcast Buildings. I just did a quick search of their website and see that these buildings are still available. Which is good to know if I ever decide to expand my collection.
These were painted with a very basic color palette, but I like the look. I need to add a little bit of detail to the roof tops at some point.
These have seen the field of battle in a variety of games and could be used in many more. 40K, (of course) Necromunda, G.A.S.L.I.G.H.T., In Her Majesty's Name, Infinity, Bolt Action, Future War Commander Skirmish, Tomorrows War, etc. I have even used them as adobe (not perfect, but works) buildings to expand my old west town for large games.
What do you use for buildings in your games? Do you prefer ruins or whole buildings?