National Institutes of Health – provides information about opportunities in laboratories currently receiving NIH funding
ScienceCareers – information about a variety of job opportunities (including academia and industry). Universities and companies can post job openings, and listings can be sorted by interest, position (postdoctoral, professor, etc.); put together by Science magazine.
AcademicCareers.com – although not science specific, provides information for positions primarily in higher education, no registration is needed to use the site
Also, many of the departments at OSU in the med center have their own post-doc/employment opportunities pages, or at least a listing of current post-docs and their contact information. Here are only a few: