Downton Abbey starts on most PBS stations tonight (Sun, 9th). It is a cut above most period dramas and while it can be overly sentimental and precious it at least keeps its internal integrity within those set bounds of a parlor drama where every eye twitch and toneless rebuff is more important than the Titanic sinking. Oh, and just look at those costumes! I prefer the ITV preview over the PBS one.
If you are a fan and looking for a Downton Abbey community check this one out on livejournal. They have fanfic, vids, episode reaction, etc. Many there are fans from the UK so obviously there will be spoilers if you have not seen it in its entirety yet.
Archive of Our Own (AO3) also has a Downton Abbey tag with a lot of fic.
The of course there is the Downton Abbey tumblr tag. As always, there could be spoilers for episodes that have aired in the UK first.