I am a big fan of Findmypast for family history research. Findmypast has images of many historical records that are not online anywhere else. For records that are also on other sites, Findmypast's indexes and transcriptions are (in my experience) more accurate. I also love the fact that we can sort the search results.
See the updated version of my article Why I Use and Recommend Findmypast, which highlights some of the outstanding features of the site.
