This is a list of manufacturers and shops of 25mm-28mm buildings and scenery pieces for a variety of settings, such as medieval villages, old west towns, etc. Since I am referring to various of these websites quite frequently I found it easier for myself to collect them in one handy post. Maybe it is useful to you, too. I will update it as I stumble across new stores and manufacturers. Despite my attempts to collec the most useful producers this list will obviously never be exhaustive. If you want to add something to the list, just let me know!
Antenociti's Workshop - Absolutely great shop with lots of windows and other parts
CT-Scenery - Some great buildings (also check out the tables under "tabletops")
Dwarven Forge - Produce cool interior buildings and accesories
Ebob Miniatures - Very cheap but good (closed) buildings. You can see some painted samples here.
em-4 - They have a great pre-painted medieval inn
ESLO (Geländestücke.de) - Great buildings and terrain. They also do commission work
Fat Spider Casting - Nice resin buildings. Not the best of detail but the prices are accordingly affordable.
Forge World - High quality resin scenery. Unfortunately the Warhammer range is somewhat limited. You can read reviews of some of the buildings on my blog: Stable, Well, Bloomery and Forge.
Frontline Wargaming - Few buildings and less pictures
Grand Manner - The ECW range has some cool timber-framed houses.
Hovels - Has many different ranges actually
Manorhouse - Awesome buildings and complete gaming boards/dioramas, but not very cheap
Menta-Modellbau - Stone walls and other pieces (German)
Mirliton SG - Produce great accessories for diaoramas such as medieval taverns or kitchens.
Old Crow Models - Ruined buidlings
Pardulon - Building kits and scenic items such as barrels
Precision Design Models - Ruined buidlings (there is also a nice little article about their ruined houses)
Resina Planet - They have incredible high quality (and properly sized) buildings and ruins. Not cheap but well worth their price. There is a review of the Governor's Mansion on the blog.
Stronghold Terrain - Great timber frame buildings (German).
Tabletop World - Probably the best resin scenery for medieval/fantasy settings. High quality buildings at fair prices. Read an interview with them on my blog.
The2legends - Some nice doors and windows and houses.
Thomarillion - Buildings and scenery pieces, some of them are also available from Ziterdes
Wargame Scenics/Hudson & Allen - lots of medieval buildings that come at rather cheap prices. Check out the painted Fortified Farm at Harness and Array blog.
Ziterdes - LotR and medieval scenery
Ainsty - Port scenery
Brigade Games - Impressive buildings
Empress Miniatures - Nice buildings at good prices
Grand Manner - Beautiful terrain, not cheap though.
Manorhouse - Awesome buildings
Old Glory 25's - Stucco buildings
TableScape - Nice walls and a few buildings
Old Crow Models - Some decent buidlings
Crescent Root Studio - Very detailed buildings, mostly oriental
CT-Scenery - Only one asian building so far.
Curteys Miniatures - They have a few houses.
Hovels - Seems to have a range for everything
John Jenkins Designs - Very detailed high quality buildings, walls, lanterns, ...
Minis and Scenery - Some nice stuff.
Oshiro modelterrain - The Asian scenery manufacturer.
Redoubt Enterprises - Some okay stuff here
Stronghold Miniatures - The Village Green range includes Japanese Peasant Houses and a Samurai walled residence.
Arnica Real Estate - also available from this adress.
Frontline Wargaming - Buildings and interior stuff
Hovels - Yes, again
Old Glory Miniatures -
Rusus.de.vu - German store with a few buildings for various settings
Terrain Builder - This shop is going to end soon, but will return with some new offers thereafter. Let's see...
BattlefieldArchitect - great scenery for different territories (jungle, savannah, swamps & more)
Zuzzy - produces awesome gaming mats.
TerranScapes - trees, hills and other scenery
Olleys Armies - They have some very cheap palm trees in the terrain section of the shop.
Olleys Armies - The Artifacts range has a few very useful packs such as flagons, tankards and candle holders.