There's an excellent classical concert search engine for New York City:

http://brahmsy.com/f?area=NY_NYC&k=classical+music

It's my go-to site for finding upcoming live classical music in NYC these days -- it has more classical than any other website I've seen, by far. You can also search by keyword, eg "Bach" or "classical music" or "violin music".

It currently only covers the New York area though.