I go to the main site for the genre I'm interested in. For instance, if it's metal I go to metalstorm.com and look at the New Releases and Upcoming Releases categories. You can usually find a site like this for each genre. You could even start at newalbumreleases.net, which contains incomplete lists for all the genres.
For old releases find a site that does ratings. Each genre usually has these too. For instance, jazz has allaboutjazz.com/reviews, although it's navigation sucks. If you want to go back more than a few months in time change the page number in the URL.