Those interested in Mark Twain have undoubtedly used Jim Zwick's sites on Twain and anti-imperialism at the turn of the century. According to the H-Amstdy listserv, Jim Zwick passed away recently.
Shelley Fisher Fishkin's tribute at the link above says it much better than I ever could, but Jim was incredibly generous with his time and knowledge about Twain and other 19th-century figures. When I began creating websites in 1997, his were among the first I found on American literature. His site was the first site to feature Howells's "Editha," for example, and his commentary on Twain was incredibly helpful. On a personal note, we exchanged a few e-mails over the years, and he generously allowed the use of some of his scans of photographs on the American Authors site (such as the page on Charles Dudley Warner). He was a pioneer in showing what could be done with American literature on the web, and as Fishkin says, he will be missed.