The 2009 International Brass Chamber Music Festival will be held at East Carolina University on October 10-13, 2009. The festival will feature a brass quintet competition, brass ensemble competition and a brass ensemble composition contest for composers.
National Foundation for Advancement of the Arts (NFAA) - supporting young (high school age) artists
National Federation of Music Clubs - Competitions - They are heavy on the competitions and awards for piano, voice, and strings, but they do have a bit left over for orchestral instruments. See their Competition Chart that lists all awards (pdf file). They also have awards in composition, music education, music therapy, and alleluia, there is one award specifically for "French horn" - see p. 6 of the Competition chart: the Elizabeth Paris Award - $1200 for students 19-26 years old; deadline is April 1 (sheer, coincidence, no doubt).
The International Horn Competition of America
The oldest (founded 1981) most prestigious horn competition in America. It has two divisions: university and professional.
5th Annual Henri Tomasi International Competition for Woodwind Quintets