Well here's a nice resource -- and particularly appropriate given the current economic climate!
musicalchairs is a "job" board for professional musicians. Orchestra managers can post openings; musicians can search positions. Visited by 10,000 folks daily!
Here's what they say about themselves:
musicalchairs aims to provide a cost-effective means for orchestras, opera companies and conservatoires to advertise vacancies worldwide. Although musicalchairs is widely read worldwide, we respect and value national job markets. We understand how crucial it is for classical music managers to reach the best possible list of applicants so that top class appointments can be made. We recognize our responsibility to make the listings as clear as possible "
Be sure and bookmark this useful resource -- you'll find them at http://www.musicalchairs.info/