The Human Languages Page by Tyler Chambers, one of the most extensive and long-established linguistics sites.
The LINGUIST List offers links to pages about languages and language families.
Patrick Ryan's Proto-Language home page; devoted to theories about the reconstruction of a single original human language, this is a well-done page with links to fascinating speculative scholarship.
Computational Historical Linguistics Projectat the University of Pennsylvania. This project uses quantitative data to reconstruct relationships among languages; it's technical, but well worth a look.
Abdelouahid Zaid's page of resources in linguistics has many definitions and suggested readings.
A paper by Rick Horowitz on issues in language change and language contact.
A paper on the history of historical linguistics by Steen Schousboe.
Emily's Guide to historical linguistics, part of a larger site.