These are the sites I use. This list has a lot of companies that are known for kits but have useful details as well. Kits alone are in a separate list.
GCLaser Details, kits
Wild West Models Shake shingles, roofing, kits
Alkem Scale Models Tobacco plants, details, kits
AMB American Model Builders: details and kits
Blair Line Roof signs, wall signs, details, grade crossings, and kits of course
Berkshire Junction EZ Line, traffic lights, electrical stuff. All magic
BEST Prototype buildings of New England, amazing line of detail castings
Branchline Shingles, window treatments, wide line of building kits and rolling stock
BTS Great details, great kits, the place to go for a sawmill complex
BIS/CC Crow Old Builders in Scale, CC Crow, great hydrocal stuff, terrific details
Chooch Enterprises Rock sheets, car loads, building fronts, DVDs, good stuff
JL Innovative Details and signs, as well as kits
LARC Products Bill Brown: superb modeler and photographer, get his backgrounds here
Mike Rose A master of details, weathering, information
Tichy Train Group Tons of details, windows, rolling stock
VectorCut Phenomenal wood laser cut furniture and details, terrible website
SierraWest Details not currently available
Anderson Rivets and such via decals
ITT Sound Modules Sounds ranging from horns to cows, and everything in between