There are many sources for this, some on subject-specific servers (e.g. for economics jobs) some more general. Jobs are also frequently advertised though subject-specific lists (i.e. economics jobs on an economics list).
(a) Jobtrak: WWW site of employment and job resources listed on the Internet, aimed at students/alumni, (USA bias): http://www.jobtrak.com/
(b) JobHunt: comprehensive listing of job-hunting resources available via the Internet: http://www.job-hunt.org/
(c) Online Career Center: WWW: http://www.occ.com/
(d) E-Span Employment database. WWW: http://www.espan.com/
(e) Academe This Week: education jobs (see separate entry)
(f) Overseas Educational Advisers’ Professional Educators Group: see separate entry
(g) International Institute for Asian Studies (IIAS): Asian studies job list (see separate entry)
(h) International Affairs Network (see separate entry)
(i) International Arid Lands Consortium (see separate entry)
(j) IDS Yellow Mondays (see separate entry)
(k) Jobsearch: UK focused service hosting adverts and CVs. http://www.jobsearch.co.uk
(l) Jobsite: UK focused service from a number of recruitment firms. Allows submission of CVs and operates an email announcing service.
(m) JobHunter: UK focused service carrying adverts from regional newspapers. http://www.jobhunter.co.uk