The website Conference Alerts
http://www.conferencealerts.com
compiles a huge amount of events according to disciplines.

After registration the service offers free e-mailed updates of conferences matching interests, available dates and preferred destinations.

You can also add a conference to the database.

Along the same lines, different compendia of first class conferences in life science are available on Nature Reviews websites:

genetics
http://www.nature.com/nrg/info/info_conf.html

cancer
http://www.nature.com/nrc/info/info_conf.html

drug discovery
http://www.nature.com/nrd/info/info_conf.html

microbiology
http://www.nature.com/nrmicro/info/info_conf.html

molecular cell biology
http://www.nature.com/nrm/info/info_conf.html

neuroscience
http://www.nature.com/nrn/info/info_conf.html

immunology
http://www.nature.com/nri/info/info_conf.html