Brent Ozar has put together a new website that provides a simple run down of the latest service packs and cumulative updates available for each version of SQL server.
There are also links to the Microsoft download pages for the updates.
I used to check for updates using various web sources this brings it altogther in a simple to use table.
Brent is an SQL guru extraordinaire – I caught his talk on wait statistics at the SQL Bits conference in London back in March 2015 and was very impressed by his knowledge and presentation style (hands everywhere!).