Gothic Fiction at Project Gutenberg
Project Gutenberg is the first and largest single collection of free ebooks. This is their portal to gothic fiction. Out-of-copyright literature, including classics like Frankenstein, Dr. Jekyll & Mr. Hyde, Dracula, The Phantom of the Opera, and others.
The Sickly Taper
Claims to be "the world's most comprehensive and authoritative online bibliography of Gothic scholarship." Maintained by the Department of English Language, Literature, and Creative Writing at the University of Windsor.
The Gothic Experience
A great introduction to the gothic from an English professor at Brooklyn College. Originally a course website.
The e-version of an exhibition from the UVa Library that draws on the Sadlier Black Collection (a repository of "first Gothics"). Organized by critical themes.
Gothic Fiction at GoodReads.com
If you're looking for more, here's a list of gothic literature (both modern and classic) tagged by users of goodreads.com.
Try one of these phrases as a "Subject" in CatSearch.