Since someone asked me about this today, I thought I would write about it. Aaron Bertrand (blog|twitter) and I sort of have a friendly competition to be the first to blog when a new SQL Server CU is released. I don’t know of any e-mail notification service for Service Packs and Cumulative Updates from Microsoft. There used to be RSS feeds that you could subscribe to, but I don’t think they are working anymore. One valuable resource is the SQL Server Release Services Blog, but it is not always updated in a timely fashion.
The Update Center shows the latest release for each major version going back to SQL Server 2000.
These KB articles listed below are updated whenever a new CU is released, and it usually happens pretty quickly. There won’t be any more Cumulative Updates for the KB articles that are marked (retired branch).
SQL Server 2012
SQL Server 2008 R2
SQL Server 2008
To be honest, what I typically do to discover new Cumulative Updates is that I set a calendar reminder for seven weeks after the last Cumulative Update, and then I start checking those updating KB articles for the next week or so after that to find out when a new CU has been released. Microsoft is pretty mysterious when it comes to announcing new Service Packs and Cumulative Updates. Sometimes there is a big announcement, and sometimes you have to do some detective work.