Some fun brass-related websites:

Noah Gladstone's Brass Ark -  Vintage high-end brass instruments for sale, including valuable hard-to-get items.

The Conn Loyalist - Lots of interesting information and history on Conn instruments. Extensive information on instrument models and serial numbers.

Alan's Trumpet World - Features "Olds Central", with valuable information on the Olds instrument company, history, models, serial numbers, and scans of old catalogs.

HNWhite.com - Website of the H.N. White family with history, photos, catalogs, and information about the H.N. White (King instruments) company.

Trombone Chat - Discussion board and classified ads for trombone players.