Sherlockian.net: The definitive source for all things Sherlock Holmes.
221B Baker Street: Complete online version of Canon, with original illustrations.
Sacrilege: One of the largest collections of Sherlock Holmes slash available on the web.
Lyra's Holmes/Watson slash links: Complete with fanfiction recommendations and an up to date list of what's new in the H/W community.
Cox & Co.: The entry page for the livejournal community of the same name.
Slash Cotillion: Muli-fandom slash archive, with a large Sherlock Holmes section.
However Improbable: Newer archive with fanfiction, monograms and a gallery.
Holmes Slash: Yahoo group (age restricted) that focuses primarily on Holmes slash.
Adult Fanfiction: Multi-fandom archive, with a large Sherlock Holmes section.
Briabrae: Comprehensive resources for Holmesian slashers.
More to come: if you'd like to see your site listed here, please contact the webmistress.