Groups:

SCA.org : homepage for the Society for Creative Anachronism

Kingdom of Meridies

Barony of the South Downs

Barony of Bryn Madoc

Research:

Stefan's Florilegium : archives of discussions on the SCA group lists.

Artchives : many images of artwork, archived by artist.

Medieval Sourcebook : search engine for all things scholarly and Medieval.

Kingdom of Atlantia A&S : fantastic set of links for the Arts & Sciences.

Vecellio's On-line collection : images of Italian Renaissance clothing as drawn by later period artist.

Costumer's Manifesto: Italian Renaissance Clothing links galore for all things worn in Italy.

Other SCAdian's websites:

Maestro niccolo difrancesco : specializing in lovely food & brewing.

Caridoc's Miscellany : wonderful for pre-interpreted recipes.