I came upon this website the other day while doing some Pendragon-related Googling. It missed my initial sweep of online Pendragon resources when I was preparing to run the GPC because, well, it didn't exist at the time!
Unfortunately, it's apparently already lost some steam and hasn't been updated in months--but Dr. Thijs, the blog's owner, can be given a pass, since he posted a generous portion of Pendragon resources in one fell swoop. Some I've seen, some I haven't; it's nice to have everything in one place.
Of particular interest to me is the Book of Maps entry, which expands the lovely full-color Salisbury map from Greg Stafford's site and goes one further by naming every damn manor in the county. Then, just to make my day further, a separate map details the major baronetcies of Salisbury. As my campaign is currently involved in county-scale politics, this was real boon for me.
Other personal favorites include the NPC Log and Romance Log (which will come in really handy if we end up running a courtly, Lady-centric campaign for the GPC's second generation) and a great one-page summary of a battle commander's options based on the results of his battle roll (sure to get some use between now and that possible courtly campaign...).
If you're running or planning to run a Pendragon campaign (particularly if it's going to be located in Salisbury!), check this one out.