In the comments on a question about the new answer votes, Robert Cartaino said:
The dedicated site for announcements is the blog: blog.stackexchange.com.
If that does not provide enough detail, there is a "Recent Feature Changes" list: Recent feature changes to Stack Exchange
Beyond that lies meta.Stackoverflow, which gets into the really nitty-gritty, insider-baseball stuff.
Three resources to pick from, depending on your level of interest in this stuff.
